console: dummycon: export dummycon_[un]register_output_notifier

Export dummycon_[un]register_output_notifier, the fbcon code needs this
and may be build as a module.

Fixes: 83d83beb ("console/fbcon: Add support for deferred console takeover")
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>