util: add time_stamp() implementation for XBOX

Backported from UnrealEngine project.

src/util.c

@@ -86,7 +86,12 @@ char *xmpp_strtok_r(char *s, const char *delim, char **saveptr)
  */
 uint64_t time_stamp(void)
 {
-#ifdef _WIN32
+#if defined(_XBOX_ONE)
+    uint64_t SystemTime;
+    GetSystemTimeAsFileTime((FILETIME*)&SystemTime);
+    /* Convert 100 nanosec ticks to milliseconds */
+    return (SystemTime / 10000);
+#elif defined(_WIN32)
     return timeGetTime();
 #else
     struct timeval tv;