Fix GCC/Posix port compilation on FreeBSD (FreeRTOS#1239)

On FreeBSD pthread_once_t is a struct and cast is required.
Otherwise there's compilation error:
../../mocks/freertos/port.c:261:23: error: expected expression
    hSigSetupThread = PTHREAD_ONCE_INIT;
                      ^
PTHREAD_ONCE_INIT is defined as: { PTHREAD_NEEDS_INIT, NULL } on FreeBSD

Author: Jakub Tymejczyk

portable/ThirdParty/GCC/Posix/port.c

@@ -303,8 +303,8 @@ BaseType_t xPortStartScheduler( void )
         memset( ( void * ) &hSigSetupThread.__opaque, 0, sizeof( hSigSetupThread.__opaque ) );
         memset( ( void * ) &hThreadKeyOnce.__opaque, 0, sizeof( hThreadKeyOnce.__opaque ) );
     #else /* Linux PTHREAD library*/
-        hSigSetupThread = PTHREAD_ONCE_INIT;
-        hThreadKeyOnce = PTHREAD_ONCE_INIT;
+        hSigSetupThread = ( pthread_once_t ) PTHREAD_ONCE_INIT;
+        hThreadKeyOnce = ( pthread_once_t ) PTHREAD_ONCE_INIT;
     #endif /* __APPLE__*/
 
     /* Restore original signal mask. */