systemc: Don't include the scheduler in an external header file.

systemc: Don't include the scheduler in an external header file.

The scheduler is an internal detail and shouldn't be exposed to the
headers in ext. It would transitively include more headers which are
not in ext, making it not self contained.

Change-Id: I8384cde9d19363953ffd0c91e7d8d27f8f79a570
Reviewed-on: https://gem5-review.googlesource.com/c/13336
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>