Include <string.h>.

strchr() is declared in <string.h>, and we now use strchr(), so we must
include <string.h>.

Change-Id: Ie80763c10c4ad1ef85d4a83d8eacc3ea236bea56
Reviewed-on: https://code.wireshark.org/review/35052
Reviewed-by: Guy Harris <guy@alum.mit.edu>

wsutil/socket.c

@@ -13,6 +13,7 @@
 #include <config.h>
 
 #include <stdlib.h>
+#include <string.h>
 #include <errno.h>
 #include <glib.h>