src: fix compiler warning

This commit fixes a -Winconsistent-missing-override warning.

PR-URL: #17195
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: cjihrig

src/node_http2.h

@@ -554,7 +554,7 @@ class Http2Stream : public AsyncWrap,
   int ReadStop() override;
 
   // Required for StreamBase
-  int DoShutdown(ShutdownWrap* req_wrap);
+  int DoShutdown(ShutdownWrap* req_wrap) override;
 
   // Initiate a response on this stream.
   inline int SubmitResponse(nghttp2_nv* nva,