Add prototype for sigset_from_compat()

Duh.  I screwed up editing David Howells patch in commit
3f2e05e9, and the actual declaration for
the sigset_from_compat() function went missing. My bad.

Olaf Hering saved the day and noticed that I'm a moron.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>