alpha: wire up accept4 syscall

Somehow wiring up the accept4 syscall on Alpha was missed long ago.
This commit rectifies that oversight.

Signed-off-by: Michael Cree <mcree@orcon.net.nz>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>