move 'type' variable to inside an #if statement

removes an unused variable when compile without NGX_DEBUG, allows for
building with -Werror.

src/ngx_base_fetch.cc

@@ -120,7 +120,9 @@ void NgxBaseFetch::ReadCallback(const ps_event_data& data) {
   NgxBaseFetch* base_fetch = reinterpret_cast<NgxBaseFetch*>(data.sender);
   ngx_http_request_t* r = base_fetch->request();
   bool detached = base_fetch->detached();
+#if (NGX_DEBUG)  // `type` is unused if NGX_DEBUG isn't set, needed for -Werror.
   const char* type = BaseFetchTypeToCStr(base_fetch->base_fetch_type_);
+#endif
   int refcount = base_fetch->DecrementRefCount();
 
 #if (NGX_DEBUG)