libgpgme: Take into account that gnupg and gnupg2 are mutual exclusive

Avoid that libgpgme can select the gnupg package although gnupg2 has
been already selected. Both packages are mutual exclusive and shouldn't
be selected at the same time.

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>