gpg: Fix extra check for sign usage of a data signature.
* g10/sig-check.c (check_signature_end_simple):
Obviously we should not ignore a back signature here.
- Fixes-commit: 214b0077264e35c079e854a8b6374704aea45cd5
- GnuPG-bug-id: T4014
- Signed-off-by: Werner Koch <wk@gnupg.org>