qapi: Fix QAPISchemaEnumType.is_implicit() for 'QType'

Missed in commit 7264f5c.  Harmless, because nothing checks whether an
enumeration type is implicit so far.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1489582656-31133-14-git-send-email-armbru@redhat.com>
