BellProc: Send bell event on core protocol bell when requested

XKB allows to override the BellProc() ringing the 'keyboard bell':
instead an event is sent to an X client which can perform an
appropriate action.
In most cases this effectively prevents the core protocol bell
from ringing: if no BellProc() is set for the device, no attempt
is made to ring a bell.
This patch ensures that an XKB bell event is sent also when
the core protocol bell is rung end thus an appropriate action
can be taken by a client.

Signed-off-by: Egbert Eich <eich@freedesktop.org>
Acked-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Keith Packard <keithp@keithp.com>