fs/internal.h: add const to ns_dentry_operations declaration

The actual definition in fs/nsfs.c is already const.
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/internal.h

@@ -156,7 +156,7 @@ extern void mnt_pin_kill(struct mount *m);
 /*
  * fs/nsfs.c
  */
-extern struct dentry_operations ns_dentry_operations;
+extern const struct dentry_operations ns_dentry_operations;
 
 /*
  * fs/ioctl.c