qt: Make sure expiration time is interpreted as unsigned number

* lang/qt/src/qgpgmeaddexistingsubkeyjob.cpp (add_subkey): Convert
expiration time to uint_least32_t.

This fixes the corresponding test on 32-bit systems where time_t (the
return type of expirationTime()) is a signed 32-bit integer type.

• GnuPG-bug-id: T6137