python: Import reduce function in FileSystemConfig

python: Import reduce function in FileSystemConfig

Not sure if this is required due to python3 or something else, but I got
the error "NameError: name 'reduce' is not defined". This fixes that
error.

Change-Id: I2dd71674306abcad1a90311664b18b9eee29b9ac
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32374
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>