systemc: Add the deprecated sc_signal_out_if typedef.

This is needed by the regression tests.

Change-Id: I5666cf9ad445869e74edda857afd59ab7ece4f4c
Reviewed-on: https://gem5-review.googlesource.com/10955
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>