signal: trivial, fix the "timespec declared inside parameter list" warning

Fix the compile warning, do_sigtimedwait(struct timespec *) in signal.h
needs the forward declaration of timespec.

Reported-and-acked-by: Mike Frysinger <vapier.adi@gmail.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>