alpha: fix compile problem in arch/alpha/kernel/signal.c

Tssk.  Apparently Al hadn't checked commit c52c2ddc1dfa ("alpha: switch
osf_sigprocmask() to use of sigprocmask()") at all. It doesn't compile.

Fixed as per suggestions from Michael Cree.

Reported-by: Michael Cree <mcree@orcon.net.nz>
Cc: Al Viro <viro@ftp.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>