mem-cache: Fix non-virtual base destructor of Repl Entry

mem-cache: Fix non-virtual base destructor of Repl Entry

ReplaceableEntry contains a virtual method, yet its destructor
was not virtual, causing errors in some compilers.

Change-Id: I13deec843f4007d9deb924882a8d98ff6a89c84f
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/19808
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>