Add missing volatile qualifier.

Yet another silly mistake in 0709b7ee,
again found by buildfarm member castoroides.

src/backend/storage/lmgr/s_lock.c

@@ -156,7 +156,7 @@ s_lock(volatile slock_t *lock, const char *file, int line)
 
 #ifdef USE_DEFAULT_S_UNLOCK
 void
-s_unlock(slock_t *lock)
+s_unlock(volatile slock_t *lock)
 {
 #ifdef TAS_ACTIVE_WORD
 	/* HP's PA-RISC */