api: workaround for espasyncwebserver vs. esp8266webserver includes

When building single source, enums for HTTP status codes have a name conflict.
ESP8266WebServer is included only in the OTA_WEB_SUPPORT=1 and WEB_SUPPORT=0,
so guard by checking for WEB_SUPPORT before including anything related to the async variant.

code/espurna/ota_basicweb.cpp

@@ -19,6 +19,7 @@ Copyright (C) 2019-2021 by Maxim Prokhorov <prokhorov dot max at outlook dot com
 #include "ota.h"
 
 #include <ESP8266WebServer.h>
+#include <StreamString.h>
 
 namespace ota {
 namespace basic_web {