move extern C markers to dm-bht headers

C code should mark its own headers as extern, so move that logic here.

BUG=chromium:878440
TEST=build passes

Change-Id: Ic90da314e5a2820faf207eb85056f7e4aa30d9b1
Reviewed-on: https://chromium-review.googlesource.com/1207270
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
4 files changed