when built with HTTP disabled, provide a curl_formadd() function anyway to

keep the API complete at all times

lib/formdata.c

@@ -1458,4 +1458,14 @@ int main(int argc, char **argv)
 
 #endif
 
-#endif /* CURL_DISABLE_HTTP */
+#else  /* CURL_DISABLE_HTTP */
+CURLFORMcode curl_formadd(struct curl_httppost **httppost,
+                          struct curl_httppost **last_post,
+                          ...)
+{
+  (void)httppost;
+  (void)last_post;
+  return CURL_FORMADD_DISABLED;
+}
+
+#endif  /* CURL_DISABLE_HTTP */