Help mingw to find the right overload to use

* src/gpgolkeyadder/gpgolkeyadder.cpp (GpgOLKeyAdder::setupGUI,
GpgOLKeyAdder::checkAccept): Cast size of QByteArrays to size_t.

This makes the code compatible with GpgME++ 2.0. It resolves an
ambiguity between two overloads of Data() that occurs because the
unwanted Data() constructor uses gpgme_off_t instead of off_t as type
of the second argument (in GpgME++ 2.0).