libiberty: Fix psignal parameter type

This is a backport of a change that was approved for the upstream trunk.
Fix the parameter type of psignal so it matches the canonical declaration.
This fixes a build problem when trying to use a nacl-newlib rebased on the
upstream newlib trunk.

BUG= none
TEST= toolchain trybot

R=pasko@google.com,eaeltsin@google.com

Review URL: http://codereview.chromium.org/8356012
1 file changed