perf: link with pthread explicitly

Without pthread in linker flags building with GCC is failed:

> /usr/include/c++/9/thread:130: undefined reference to `pthread_create'

The patch fixes that.

Needed for tarantool/tarantool#10866

Author: ligurio

perf/CMakeLists.txt

@@ -20,5 +20,5 @@ endif()
 include_directories("${PROJECT_SOURCE_DIR}")
 
 add_executable(small.perftest small.cc)
-target_link_libraries(small.perftest small ${BENCHMARK_LIBRARIES})
+target_link_libraries(small.perftest small ${BENCHMARK_LIBRARIES} pthread)
 target_include_directories(small.perftest PUBLIC ${BENCHMARK_INCLUDE_DIRS})