dev: Stop "using namespace std"

dev: Stop "using namespace std"

Change-Id: I317df9b566936445c3a02c28ed37146a16610454
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39538
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>