[gnome-settings-daemon] Add missing include for NULL.

[Antoine Jacoutot <ajacoutot src gnome org>]

commit 7936ac8d2d6724c472a81f84e3aa6fcf49aec7f5
Author: Antoine Jacoutot <ajacoutot gnome org>
Date:   Fri Mar 29 15:32:49 2013 +0100

    Add missing include for NULL.
    
    To prevent the following error on some OS:
    error: 'NULL' undeclared (first use in this function)
    
    https://bugzilla.gnome.org/show_bug.cgi?id=696856

 plugins/power/gsd-backlight-linux.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/plugins/power/gsd-backlight-linux.c b/plugins/power/gsd-backlight-linux.c
index bbaf490..37e0f00 100644
--- a/plugins/power/gsd-backlight-linux.c
+++ b/plugins/power/gsd-backlight-linux.c
@@ -25,6 +25,8 @@
 
 #include "gsd-backlight-linux.h"
 
+#include <stdlib.h>
+
 #ifdef HAVE_GUDEV
 #include <gudev/gudev.h>