Add a dummy statement to the beginning of the pthread_cleanup_pop() macro

to allow a call of the macro to be labelled as in:

label:
  pthread_cleanup_pop();

Reviewed by:	imp
MFC after:	3 days

include/pthread.h

@@ -175,6 +175,7 @@ int		pthread_barrierattr_setpshared(pthread_barrierattr_t *, int);
 			{
 
 #define		pthread_cleanup_pop(execute)					\
+				(void)0;					\
 			}							\
 			__pthread_cleanup_pop_imp(execute);			\
 		}