gtestutils: Include stdlib.h for exit function

This fixes warning: implicit declaration of function 'exit'

glib/gtestutils.h

@@ -28,6 +28,7 @@
 #include <glib/gerror.h>
 #include <glib/gslist.h>
 #include <errno.h>
+#include <stdlib.h>
 #include <string.h>
 
 G_BEGIN_DECLS