config: Fix missing include in fs.py

config: Fix missing include in fs.py

Bugfix for Elastic Traces

This patch fixes the bug when elastic traces are used:

    build/ARM/gem5.opt \
    configs/example/fs.py \
    --cpu-type=arm_detailed \
    --num-cpu=1 \
    --mem-type=SimpleMemory \
    --mem-size=512MB \
    --mem-channels=1 \
    --caches \
    --elastic-trace-en \
    --data-trace-file=data.proto.gz \
    --inst-trace-file=inst.proto.gz \
    --machine-type=VExpress_EMM \
    --dtb-filename=vexpress.aarch32.ll_20131205.0-gem5.1cpu.dtb \
    --kernel=vmlinux.aarch32.ll_20131205.0-gem5 \
    --disk-image=linux-aarch32-ael.img

NameError: global name 'CpuConfig' is not defined

Signed-off by: Jason Lowe-Power <jason@lowepower.com>