core: Tell gpg that we want to verify signed data

* src/engine-gpg.c (gpg_verify): Add "--verify" to command line.

Since GnuPG 2.1.16 --verify writes the signed data to the file specified
by --output. Explicitly telling gpg that we want to verify signed data
frees gpg from guessing what we want and avoids the corresponding
warning "no command supplied. Trying to guess what you mean ..."

• GnuPG-bug-id: T6907