core: Use fully correct command args for gpg --verify.

* src/engine-gpg.c (gpg_verify): Supply --verify for fixed gpg
versions.

With 2.1.something we print a warning if no explict command was
given. Since 2.1.16 this has been fixed to also allow the combination
of --verify and --output. Thus we now always use "--verify".

• Signed-off-by: Werner Koch <wk@gnupg.org>