mem-cache: Fix missing header in associative set

mem-cache: Fix missing header in associative set

Add missing intmath header to AssociativeSet, so that isPowerOf2 can
be used.

error: there are no arguments to 'isPowerOf2' that depend on a template
parameter, so a declaration of 'isPowerOf2' must be available

Change-Id: Ib2b194f9e71284ee439786bdb76d99858e57e2f5
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22444
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>