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 <[email protected]> Auto-Submit: Mike Frysinger <[email protected]> Reviewed-by: Gwendal Grignou <[email protected]> Commit-Queue: Gwendal Grignou <[email protected]>