proto: Add a missing std:: in protoio.cc.

Also include string which that file uses.

Change-Id: I79cd9c88652abdd1d16b3368dd37268509f4d3f0
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/60310
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
1 file changed