cpu: Allow PcCountTracker to compile in NULL ISA

While the PcCountTracker isn't necessary in the NULL ISA, the
structure of the standard library requires us to have it built
when running the replacement policy tests, which should fix
these tests failing within the nightlies at the moment.

Change-Id: I225b7923f2a11d351c24bdceba3ded4ed2b3bc87
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/68597
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
1 file changed