systemc: Fix clang9 linker error

systemc: Fix clang9 linker error

Likely a compiler bug, but if this function is allowed to be inlined,
clang9 throws a linker error. Fix this error by making sure the function
isn't inlined.

Change-Id: I4bfade889796915e7bb4b224eafa6e72d4ec59da
Issue-on: https://gem5.atlassian.net/projects/GEM5/issues/GEM5-597
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29394
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>