gpg: Extra check for sign usage when verifying a data signature.

* g10/sig-check.c (check_signature_end_simple): Check sign usage.

Without this patch the signature verification fails only due to the
missing back signature. This check better explains what went wrong.

• GnuPG-bug-id: T4014
• Signed-off-by: Werner Koch <wk@gnupg.org>