systemc: Add a missing "const" on one of the sc_event operators.

Change-Id: I073ccb0f2c6d1bfebadb95869d6acf7f4ce565af
Reviewed-on: https://gem5-review.googlesource.com/c/14135
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>
2 files changed