cpp: Ensure correct expiration time on 32-bit arch with 64-bit time_t

* lang/cpp/src/key.cpp (Subkey::expirationTime): Cast away the
signedness of _gpgme_subkey.expires before casting to time_t.

_gpgme_subkey.expires stores the expiration as long int although the
expiration is always an unsigned value. Casting the value to unsigned
long int before casting it to time_t ensures that we get the correct
expiration value for 64-bit time_t even on 32-bit systems. With signed
32-bit time_t we still get a negative value as before.

• GnuPG-bug-id: T7627