lockdep: Fix BFS build

Fix:

  kernel/built-in.o: In function `lockdep_stats_show':
  lockdep_proc.c:(.text+0x48202): undefined reference to `max_bfs_queue_depth'

As max_bfs_queue_depth is only available under
CONFIG_PROVE_LOCKING=y.

Cc: Ming Lei <tom.leiming@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/lockdep_proc.c

@@ -411,8 +411,10 @@ static int lockdep_stats_show(struct seq_file *m, void *v)
 			max_lockdep_depth);
 	seq_printf(m, " max recursion depth:           %11u\n",
 			max_recursion_depth);
+#ifdef CONFIG_PROVE_LOCKING
 	seq_printf(m, " max bfs queue depth:           %11u\n",
 			max_bfs_queue_depth);
+#endif
 	lockdep_stats_debug_show(m);
 	seq_printf(m, " debug_locks:                   %11u\n",
 			debug_locks);