Fix build with gpgme >=1.18.0

In previous versions GpgmeppConfig.cmake had a find_package(QGpme) call for compatibility reasons. It was dropped for 1.18.0 though [1].

[1] https://git.gnupg.org/cgi-bin/gitweb.cgi?p=gpgme.git;a=commit;h=6e45da30ae787dd45836e55d34f7aa4e001b641c