Fix all -Wimplicit-function-declaration errors for GCC 14+

[P4o1o]

Fixes #33

Problem

Starting with GCC 14, -Wimplicit-function-declaration was promoted from
warning to error by default. This breaks the OpenVML build on all modern
distributions shipping GCC 14+ (Fedora 40+, CentOS Stream 10, Ubuntu 24.10+,
Arch, etc.).

On GCC ≤ 13 these were only warnings, so the build would succeed and all
tests passed — masking the underlying bugs.

This PR fixes all three implicit declaration issues at once, so that
the build actually succeeds end-to-end on GCC 14+.

Changes

1. Fix typo sdqr_k → dsqr_k (include/openvml_kernel.h)

The double-precision sqr kernel was declared as sdqr_k instead of dsqr_k.
Every other kernel in the project follows the naming convention
s<func>_k (float), d<func>_k (double), c<func>_k (complex float),
z<func>_k (complex double). For example: sadd_k/dadd_k,
ssub_k/dsub_k, spow_k/dpow_k.

The macros in openvml_macros.h correctly define DSQR_K as dsqr_k,
but the prototype in openvml_kernel.h declared sdqr_k — a simple
transposition of the "s" and "d" prefixes.

-void OpenVML_FUNCNAME(sdqr_k)(VMLLONG n, double * a, double * b, double * y, double * z, double * other_params);
+void OpenVML_FUNCNAME(dsqr_k)(VMLLONG n, double * a, double * b, double * y, double * z, double * other_params);

2. Fix exp10/exp10f implicit declarations (openvml_common.h + include reordering)

exp10() and exp10f() are GNU extensions. Their prototypes are only
exposed by <math.h> when _GNU_SOURCE is defined. The project never
defined this macro, so the prototypes were invisible to the compiler.

Approach: rather than sprinkling #define _GNU_SOURCE in each individual
source file, I defined it once in include/openvml_common.h, which is
the project-wide common header included (directly or transitively) by all
source files. This is the cleanest and most maintainable solution — any
future GNU extension usage will also be covered.

Additionally, several files in kernel/generic/ and reference/ had
<math.h> included before "openvml_kernel.h" (which transitively
includes openvml_common.h). This meant _GNU_SOURCE was not yet defined
when <math.h> was processed. I reordered the includes so that the
project's own headers come first:

-#include <math.h>
-#include "openvml_kernel.h"
+#include "openvml_kernel.h"
+#include <math.h>

This follows the widely accepted best practice of including project/local
headers before system headers, ensuring that project-wide defines are
active before any system header is parsed.

Affected files:

• include/openvml_common.h (added #define _GNU_SOURCE)
• kernel/generic/*.c (reordered includes where needed)
• reference/vexp10.c (reordered includes)

3. Fix get_cycles implicit declaration (test/vml_util.c)

get_cycles() is defined in test/cycle.c and declared in test/cycle.h,
but test/cycle.h had a typo in the preprocessor definition.

 #ifdef _OPENVML_CYCLE_H_
+#ifndef _OPENVML_CYCLE_H_

Same error in test/openvml_timer.h

 #ifdef _OPENVML_TIMER_H_
+#ifndef _OPENVML_TIMER_H_

Why a single PR

All three issues produce the same compiler error (-Wimplicit-function-declaration)
and have the same root cause: GCC 14 turning this from a warning into an error.
Fixing only one would still leave the build broken on GCC 14+, so it makes
sense to address them together. The issue #33 is not truly resolved unless
all three are fixed.

Testing

• Clean build (make clean && make) completes with zero
  -Wimplicit-function-declaration warnings/errors
• All 63 run_vml_test tests pass
• Tested on x86_64 Linux, GCC 13.3 (where these are still warnings,
  confirming backward compatibility)

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