mem-cache: Fix copy ellision on base compressor

Newer compiler versions have a problem with this move as
it prevents copy elision.

Change-Id: I802703df12e171d6a377b673d0ad7e202456b516
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33835
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
1 file changed