block: pg: add header include guard

Add a header include guard just in case.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/uapi/linux/pg.h

@@ -35,6 +35,9 @@
 
 */
 
+#ifndef _UAPI_LINUX_PG_H
+#define _UAPI_LINUX_PG_H
+
 #define PG_MAGIC	'P'
 #define PG_RESET	'Z'
 #define PG_COMMAND	'C'
@@ -61,4 +64,4 @@ struct pg_read_hdr {
 
 };
 
-/* end of pg.h */
+#endif /* _UAPI_LINUX_PG_H */