systemc: Move a function after the class it uses internally.

systemc: Move a function after the class it uses internally.

The class was defined, but only later in the file. By putting the
function definition later, clang stops reporting an error.

Change-Id: Id4dd1ec3f3a06f4d1dc10ef4ff8c545d98a6ae12
Reviewed-on: https://gem5-review.googlesource.com/c/13877
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>