WIFEXITED and friends are defined in sys/wait.h
Fixes a build failure on BSD.

https://bugs.gentoo.org/show_bug.cgi?id=309443
https://bugzilla.gnome.org/show_bug.cgi?id=605460

--- metacity-2.28.1/src/core/delete.c.old	2010-03-15 07:46:52.000000000 +0100
+++ metacity-2.28.1/src/core/delete.c	2010-03-15 07:47:12.000000000 +0100
@@ -32,6 +32,7 @@
 #include "workspace.h"
 
 #include <sys/types.h>
+#include <sys/wait.h> /* WIF* macros */
 #include <signal.h>
 #include <unistd.h>
 #include <errno.h>