sim: Fix compilation error for debug builds.

sim: Fix compilation error for debug builds.

https://gem5-review.googlesource.com/c/public/gem5/+/39537 removed the
implicit use of the std:: namespace. This change adds a missing
namespace specifier for debug builds.

Change-Id: I1d70602a870a25f68d7fec4b4931ba7cbbb4f4ca
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39760
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>