systemc: Make an include in src/systemc/ext use a relative path.

systemc: Make an include in src/systemc/ext use a relative path.

The includes in src/systemc/ext are supposed to use relative paths so
that they can be included in other bodies of code which aren't based
in gem5 and don't share it's -I-s, or potentially even have access to
anything outside of src/systemc/ext.

Change-Id: Icde457329c2c4ab4689221015bfcfe2ff8b051f0
Reviewed-on: https://gem5-review.googlesource.com/c/16262
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>