mem-cache: Assert Entry inherits from QueueEntry in Queue

mem-cache: Assert Entry inherits from QueueEntry in Queue

Queue has several assumptions regarding its template parameter,
so make sure they are fulfilled by forcing Entry to be derived
from QueueEntry.

Change-Id: I0203a62aec00c04ac89e9674d86a44a07f9f13ab
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17529
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>