Fix test for existence of the signing_time

* src/cms.c (build_signed_data_attributes): Fix test.

The test predates the introduction of the ksba_isotime_t which
orginally was a standard time_t. Obviously I missed to adjust this
test back in 2003.

• GnuPG-bug-id: T4213
• Signed-off-by: Werner Koch <wk@gnupg.org>