alpha: fix TIF_NOTIFY_SIGNAL handling

[ Upstream commit e2c7554c ]

it needs to be added to _TIF_WORK_MASK, or we might not reach
do_work_pending() in the first place...

Fixes: 5a9a8897 "alpha: add support for TIF_NOTIFY_SIGNAL"
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>