Making fs/minix/minix.h double including safe

I happened to find that fs/minix/minix.h doesn't guard double include.

Yes, I know this never cause something destructive because this is
self-evidence that no source file includes minix.h twice, but I think
fixing this is better than disregarding it.
Signed-off-by: Hitoshi Mitake <mitake@dcl.info.waseda.ac.jp>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/minix/minix.h

+#ifndef FS_MINIX_H
+#define FS_MINIX_H
+
 #include <linux/fs.h>
 #include <linux/pagemap.h>
 #include <linux/minix_fs.h>
@@ -86,3 +89,5 @@ static inline struct minix_inode_info *minix_i(struct inode *inode)
 {
 	return list_entry(inode, struct minix_inode_info, vfs_inode);
 }
+
+#endif /* FS_MINIX_H */