systemc: Put systemc headers in the include path when supported.

systemc: Put systemc headers in the include path when supported.

When the native systemc API support is built in, put the systemc and
systemc.h header files in the include path so they can be used as
normal. We don't want any external systemc headers being included and
getting mixed in with our local ones.

Change-Id: I5fc01ff5f069cfadb7c19a9dead13e7ce7272976
Reviewed-on: https://gem5-review.googlesource.com/c/14397
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>