Mark some more variables as static or include the appropriate header

Detected by clang's -Wmissing-variable-declarations.

From: Andres Freund <andres@anarazel.de>

src/backend/storage/lmgr/s_lock.c

@@ -19,6 +19,7 @@
 #include <unistd.h>
 
 #include "storage/s_lock.h"
+#include "storage/barrier.h"
 
 slock_t		dummy_spinlock;