systemc: Exclude a test which probably could never compile?

Part of this test just seems to be missing, so I'm not sure how it
would ever actually work.

Change-Id: I5350fea1dfd288e34e01afcd3780b2ffc3312344
Reviewed-on: https://gem5-review.googlesource.com/11260
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>