staging: lustre: fix sparse warning

Sparse reports:

drivers/staging/lustre/lustre/obdclass/obd_mount.c:1284:6: warning:
 symbol 'lustre_kill_super' was not declared. Should it be static?

Fix this warning by making lustre_kill_super static.
It is not used outside this file.
Signed-off-by: NTal Shorer <tal.shorer@gmail.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/lustre/lustre/obdclass/obd_mount.c

@@ -1286,7 +1286,7 @@ struct dentry *lustre_mount(struct file_system_type *fs_type, int flags,
 	return mount_nodev(fs_type, flags, &lmd2, lustre_fill_super);
 }
 
-void lustre_kill_super(struct super_block *sb)
+static void lustre_kill_super(struct super_block *sb)
 {
 	struct lustre_sb_info *lsi = s2lsi(sb);