mem-cache: Make findVictim non-const

mem-cache: Make findVictim non-const

In order to acquire stats when a victim is found,
findVictim must be made const.

Change-Id: I493c7849f07625c90b2b95fd220f50751f4d0f52
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22604
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>