constify string pointers to save a little space

No need for these pointers to be writable.

BUG=None
TEST=CQ passes

Change-Id: Ie76e36fb5e639cb77d895e377109bb81da7ad105
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/rootdev/+/5544863
Tested-by: Mike Frysinger <vapier@chromium.org>
Auto-Submit: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Gwendal Grignou <gwendal@chromium.org>
Commit-Queue: Gwendal Grignou <gwendal@chromium.org>