common: Pass XDG_SESSION_TYPE and QT_QPA_PLATFORM envvars to pinentry

* common/session-env.c (stdenvnames): Add XDG_SESSION_TYPE and
QT_QPA_PLATFORM.

On Unix systems (except Darwin), Qt uses those two environment
variables additionally to DISPLAY and WAYLAND_DISPLAY to figure out
whether to use X11 or Wayland. For example, QT_QPA_PLATFORM needs
to be set to "wayland" to make Qt use Wayland on Gnome.

• GnuPG-bug-id: T3659