[mutter] meta-backend: include stdlib.h

[Ray Strode <halfline src gnome org>]

commit cf3ee327a069756fec1ee8403980719e6d4bc68a
Author: Ray Strode <rstrode redhat com>
Date:   Fri Nov 6 23:10:41 2015 -0500

    meta-backend: include stdlib.h
    
    Otherwise build fails with missing declaration
    warning for exit().

 src/backends/meta-backend.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/src/backends/meta-backend.c b/src/backends/meta-backend.c
index 79e5f2e..8197478 100644
--- a/src/backends/meta-backend.c
+++ b/src/backends/meta-backend.c
@@ -24,6 +24,8 @@
 
 #include "config.h"
 
+#include <stdlib.h>
+
 #include <meta/meta-backend.h>
 #include "meta-backend-private.h"
 #include "meta-input-settings-private.h"