src: fix -Winconsistent-missing-override warning

Fix a compiler warning that was introduced in commit 4db1bc8f205
("http2: allocate on every chunk send") by adding an `override` keyword.

PR-URL: https://github.com/nodejs/node/pull/16726
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

src/node_http2.h

@@ -431,7 +431,7 @@ class Http2Session : public AsyncWrap,
                   const SubmitTrailers& submit_trailers) override;
 
   void Send(WriteWrap* req, char* buf, size_t length) override;
-  WriteWrap* AllocateSend();
+  WriteWrap* AllocateSend() override;
 
   int DoWrite(WriteWrap* w, uv_buf_t* bufs, size_t count,
               uv_stream_t* send_handle) override;