Fix check for primary key

Comparing the fingerprint pointers doesn't work because
Key::primaryFingerPrint() points to a copy of the fingerprint of the
primary subkey. Comparing the keyID pointers does work but it uses
internal knowledge about Key::keyID(). The next version of GpgME++
brings Subkey::isPrimaryKey() which makes the check easy and fast.

• GnuPG-bug-id: T8321