[SYCL][PI] Restore find_package(Threads) to fix build issue in some env (#6059)

Fix regression after #6023

Author: smaslov-intel

sycl/plugins/level_zero/CMakeLists.txt

@@ -103,6 +103,8 @@ if (SYCL_ENABLE_XPTI_TRACING)
   set(XPTI_PROXY_SRC "${CMAKE_SOURCE_DIR}/../xpti/src/xpti_proxy.cpp")
 endif()
 
+find_package(Threads REQUIRED)
+
 add_sycl_plugin(level_zero
   SOURCES
     "${sycl_inc_dir}/CL/sycl/detail/pi.h"
@@ -114,6 +116,7 @@ add_sycl_plugin(level_zero
     ${XPTI_PROXY_SRC}
   LIBRARIES
     "${LEVEL_ZERO_LOADER}"
+    Threads::Threads
 )
 
 find_package(Python3 REQUIRED)