fs: 9p: cache.h: Add #define of include guard

The include file was intended to have an include guard, but the #define
part is missing.
Signed-off-by: Tzvetelin Katchov <katchov@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/9p/cache.h

@@ -21,6 +21,7 @@
  */
 
 #ifndef _9P_CACHE_H
+#define _9P_CACHE_H
 #ifdef CONFIG_9P_FSCACHE
 #include <linux/fscache.h>
 #include <linux/spinlock.h>