gpg: Handle critical marked 'Reason for Revocation'.

* g10/parse-packet.c (can_handle_critical): Add
SIGSUBPKT_REVOC_REASON.

Some software seems to mark that subpacket as criticial. Although gpg
has no special treatment for a revocation reasons (except for
--list-packets) we can accept a criticial marked anyway. There are no
mandatary rules specified on how to handle a revocation reason.

• Signed-off-by: Werner Koch <wk@gnupg.org>