systemc: Change the type of a loop counter to avoid a warning.

systemc: Change the type of a loop counter to avoid a warning.

g++ complained about comparing an signed int loop counter with the
return value of a size() function. This change changes it to an
unsigned to make g++ happy/quiet.

Change-Id: I28fa79c448465b24d77b5623860f9b991f313561
Reviewed-on: https://gem5-review.googlesource.com/c/16286
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>