src: fix -Winconsistent-missing-override warning

Fix a compiler warning that was introduced in commit 4db1bc8
("http2: allocate on every chunk send") by adding an `override` keyword.

PR-URL: #16726
Reviewed-By: Anatoli Papirovski <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Daniel Bevenius <[email protected]>
Reviewed-By: Franziska Hinkelmann <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Michael Dawson <[email protected]>
Reviewed-By: Michaël Zasso <[email protected]>

src/node_http2.h

@@ -444,7 +444,7 @@ 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;