build: Require same major/minor version of gpgme as version of gpgmepp
* CMakeLists.txt: Set GPGME_REQUIRED_VERSION to VERSION_MAJOR.VERSION_MINOR.0.
Often API changes in gpgme are reflected in gpgmepp. Therefore, always
require the same major/minor version of gpgme for gpgmepp.
- GnuPG-bug-id: T7262