mem: Added missing override to cache_blk function

mem: Added missing override to cache_blk function

This was causing a compilation warning/error when compiling with clang.

Change-Id: Ic6cf59c002656ba2ab05d8b58766613c289e7db0
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36695
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>