meson: ensure dbus-display generated code is built before other units

meson: ensure dbus-display generated code is built before other units

It's simply by luck that dbus-display header is built first before the
other units using it.

With sourceset, I can't find an easier way out than declaring an extra
dependency for dbus-display1 generate code.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
(cherry picked from commit 1222070e772833c6875e0ca63565db12c22df39e)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>