Remove unnecessary #include "spin.h" from shmem.h

Commit 6b8238cb6a removed the last usage of slock_t from the
file. proc.c was relying the indirect #include, so add it to proc.c
directly.

src/include/storage/proc.h

@@ -22,6 +22,7 @@
 #include "storage/pg_sema.h"
 #include "storage/proclist_types.h"
 #include "storage/procnumber.h"
+#include "storage/spin.h"
 
 /* Avoid including clog.h here */
 typedef int XidStatus;

src/include/storage/shmem.h

@@ -21,7 +21,6 @@
 #ifndef SHMEM_H
 #define SHMEM_H
 
-#include "storage/spin.h"
 #include "utils/hsearch.h"