Compilation success with pthreads enabled

- The project can be compiled with pthread enabled on opencv build
	- FIXME: Needed to add a call to pthread_cond_init() in the main component for the linker to work.
	The pthread_cond implementation is done in esp-idf/components/pthreads/
- The optimization hasn't yet enhanced the benchmark performances, hasn't found why for now

modules/core/src/parallel_impl.cpp

@@ -7,13 +7,7 @@
 #include "parallel_impl.hpp"
 
 #ifdef HAVE_PTHREADS_PF
-#ifdef __cplusplus
-extern "C" {
-#endif
 #include <pthread.h>
-#ifdef __cplusplus
-}
-#endif
 
 #include <opencv2/core/utils/configuration.private.hpp>