systemc: Add a missing override.

A recent change accidentally left off the override, upsetting gcc.

Change-Id: I78cf1969aa6ac462539a2793a8a91dea32002f3a
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34756
Maintainer: Gabe Black <gabeblack@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/systemc/core/scheduler.hh

@@ -160,7 +160,7 @@ class Scheduler
         Tick targeted_when;
         Scheduler* parent_scheduler;
         ScEvents events;
-        void process();
+        void process() override;
 
       protected:
         void