Include <sys/wait.h> for WIFEXITSTATUS/WIFEXITED

Exit status macros are specified by POSIX to be in <sys/wait.h>.
Fixes compilation on Solaris.

Reported by Brian Cameron
https://bugzilla.gnome.org/show_bug.cgi?id=612506

src/core/delete.c

@@ -32,6 +32,7 @@
 #include "workspace.h"
 
 #include <sys/types.h>
+#include <sys/wait.h>
 #include <signal.h>
 #include <unistd.h>
 #include <errno.h>