gpg: Fix showpref to list AEAD feature.

* g10/keyedit.c (show_prefs): Show 'AEAD' if flags.aead is set.

The terse 'pref' command in the key editor correctly shows '[aead]'
if the uid->flags.aead is set, but the more verbose 'showpref'
command does not, due to an inverted condition check.

• Signed-off-by: Damien Goutte-Gattat <dgouttegattat@incenp.org>