locking/lockdep: Fix print_collision() unused warning

Fix this:

  kernel/locking/lockdep.c:2051:13: warning: ‘print_collision’ defined but not used [-Wunused-function]
  static void print_collision(struct task_struct *curr,
              ^
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1459759327-2880-1-git-send-email-bp@alien8.deSigned-off-by: Ingo Molnar <mingo@kernel.org>

kernel/locking/lockdep.c

@@ -1999,6 +1999,7 @@ static inline int get_first_held_lock(struct task_struct *curr,
 	return ++i;
 }
 
+#ifdef CONFIG_DEBUG_LOCKDEP
 /*
  * Returns the next chain_key iteration
  */
@@ -2069,6 +2070,7 @@ static void print_collision(struct task_struct *curr,
 	printk("\nstack backtrace:\n");
 	dump_stack();
 }
+#endif
 
 /*
  * Checks whether the chain and the current held locks are consistent