base: Using scoped string in DPRINTFNR

base: Using scoped string in DPRINTFNR

We would otherwise need to add a using namespace std wherever we
use DPRINTFNR.

Change-Id: I30bf9ba474408133abded66141f6dc96dfdba8d6
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21821
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>