src: fix -Winconsistent-missing-override warning

Fix a compiler warning that was introduced in commit 4db1bc8f
("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>