CMakeLists.txt - GPGME_REQUIRED_VERSION is 1.21.0

If the gpgme version is less than 1.21.0 then
the settings for the gpgme-compat are never set.

For example, when using 1.20 QGPGME_HAS_TOLOGSTRING
is not set and that leads to a compile error about
redefining toLogString.