fix leak in Windows version of pthread_mutex_free

Free the lock pointer in addition to the critical section. Thanks to
martinkucera74 on github.

include/compat/pthread.h

@@ -106,6 +106,7 @@ static inline int
 pthread_mutex_destroy(pthread_mutex_t *mutex)
 {
 	DeleteCriticalSection(mutex->lock);
+	free(mutex->lock);
 	return 0;
 }