alpha: fix TIF_NOTIFY_SIGNAL handling

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>

arch/alpha/include/asm/thread_info.h

@@ -75,7 +75,7 @@ register struct thread_info *__current_thread_info __asm__("$8");
 
 /* Work to do on interrupt/exception return.  */
 #define _TIF_WORK_MASK		(_TIF_SIGPENDING | _TIF_NEED_RESCHED | \
-				 _TIF_NOTIFY_RESUME)
+				 _TIF_NOTIFY_RESUME | _TIF_NOTIFY_SIGNAL)
 
 /* Work to do on any return to userspace.  */
 #define _TIF_ALLWORK_MASK	(_TIF_WORK_MASK		\