Get rid of implicit declaration of function warning

--- exp_trap.c.orig	Thu Sep  9 18:39:22 2010
+++ exp_trap.c	Thu Sep  9 18:39:35 2010
@@ -13,6 +13,7 @@ would appreciate credit if this program or parts of it
 #include <stdio.h>
 #include <signal.h>
 #include <sys/types.h>
+#include <string.h>
 
 #ifdef HAVE_SYS_WAIT_H
 #include <sys/wait.h>