fix compiler warning: enumerated type mixed with another type

lib/easy.c

@@ -1096,7 +1096,7 @@ CURLcode curl_easy_recv(CURL *curl, void *buffer, size_t buflen, size_t *n)
     return CURLE_AGAIN;
 
   if(ret1 != CURLE_OK)
-    return ret1;
+    return (CURLcode)ret1;
 
   *n = (size_t)n1;