Put back required #include

Fix for dbbca2c: "storage/shmem.h" is required with
-Dspinlocks=false.

Author: petere

src/backend/storage/lmgr/spin.c

@@ -23,6 +23,7 @@
 #include "postgres.h"
 
 #include "storage/pg_sema.h"
+#include "storage/shmem.h"
 #include "storage/spin.h"