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 <68c46a606457643eab92053c1c05574abb26f861@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32374
Reviewed-by: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>
Maintainer: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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