sim: Add M5_VAR_USED to var used in dprint

Change-Id: I8f8654b8546ee8df3d4acd1ccbc5080ad38764c1
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/30896
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/sim/system.cc

@@ -179,7 +179,7 @@ System::Threads::quiesce(ContextID id)
 {
     auto &t = thread(id);
 #   if THE_ISA != NULL_ISA
-    BaseCPU *cpu = t.context->getCpuPtr();
+    BaseCPU M5_VAR_USED *cpu = t.context->getCpuPtr();
     DPRINTFS(Quiesce, cpu, "quiesce()\n");
 #   endif
     t.quiesce();