http2.h: provide empty macros for non-http2 builds

lib/http2.h

@@ -38,6 +38,8 @@ CURLcode Curl_http2_request_upgrade(Curl_send_buffer *req,
                                     struct connectdata *conn);
 void Curl_http2_switched(struct connectdata *conn);
 #else /* USE_NGHTTP2 */
+#define Curl_http2_init(x)
+#define Curl_http2_send_request(x)
 #define Curl_http2_request_upgrade(x,y) CURLE_OK
 #define Curl_http2_switched(x)
 #endif