Add a cast to satisfy mac buildbot.

Change-Id: I625b025d3f8a57812512497c6104977ae5d10232
Reviewed-on: https://code.wireshark.org/review/1298
Reviewed-by: Evan Huus <eapache@gmail.com>

epan/wmem/wmem_map.c

@@ -220,7 +220,7 @@ guint32
 wmem_strong_hash(const guint8 *buf, const size_t len)
 {
     const guint8 * const end = (const guint8 *)buf + len;
-    guint32 hash = preseed + len;
+    guint32 hash = preseed + (guint32)len;
 
     while (buf < end) {
         hash += (hash << 10);