build: add typecast to avoid warning

There is an implicit conversion from "unsigned long" to "long";
rounding, sign extension, or loss of accuracy may result.

lib/pingpong.c

@@ -322,7 +322,7 @@ CURLcode Curl_pp_readresp(curl_socket_t sockfd,
        */
       DEBUGASSERT((ptr+pp->cache_size) <= (buf+BUFSIZE+1));
       memcpy(ptr, pp->cache, pp->cache_size);
-      gotbytes = pp->cache_size;
+      gotbytes = (ssize_t)pp->cache_size;
       free(pp->cache);    /* free the cache */
       pp->cache = NULL;   /* clear the pointer */
       pp->cache_size = 0; /* zero the size just in case */