qt: Ensure that we check the correct paths

* lang/qt/src/QGpgmeConfig-w32.cmake.in.in,
lang/qt/src/QGpgmeQt6Config-w32.cmake.in.in: Get paths to check from
the target properties.

This avoids the error present in an older version where the files to
check used the correct path, but a wrong path of the DLL was set in the
target properties.