Added header for strdup

Author: Simon Hofmann

src/xdisplay.c

@@ -1,6 +1,7 @@
 #include "xdisplay.h"
 #include <stdio.h> /* For fputs() */
 #include <stdlib.h> /* For atexit() */
+#include <string.h> /* For strdup() */
 
 static Display *mainDisplay = NULL;
 static int registered = 0;