MINOR: stick-table: add sc-add-gpc() to http-after-response

MINOR: stick-table: add sc-add-gpc() to http-after-response

sc-add-gpc() was implemented in 5a72d03 ("MINOR: stick-table:
implement the sc-add-gpc() action")

This new action was exposed everywhere sc-inc-gpc() is available,
except for http-after-response.
But there doesn't seem to be a technical constraint that prevents us from
exposing it in http-after-response.
It was probably overlooked, let's add it.

No backport needed, unless 5a72d03 ("MINOR: stick-table: implement the
sc-add-gpc() action") is being backported.