mem-cache: Modify compressor to appease newer compilers

mem-cache: Modify compressor to appease newer compilers

The type of the local unique_ptr variable was different from the return type.

In C++11 because of such difference, a copy-ellision would not be possible,
and that required the use of a std::move.

In C++14 the restriction of same types being required was removed, so
std::move would not be needed anymore.

With the addition of the -Wredundant-move warning in newer compilers, having
the std::move on the return became an issue, breaking compilation.

Change-Id: I45d18dfc500bb5db5fe360814feb91853c735a19
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22403
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>