fmap.h: Add `extern "C"` to make it compatible to C++.

fmap.h is a C header file. We add `extern "C"` to let C++ code be able
to include it.

BUG=none
TEST=emerge flashmap
     successfully build a c++ program which includes fmap.h

Change-Id: I0317ddc9ce7182d69899d7756688ee7dcaded26a
Reviewed-on: https://chromium-review.googlesource.com/505677
Commit-Ready: Nicolas Boichat <drinkcat@chromium.org>
Tested-by: Chih-Yu Huang <akahuang@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
1 file changed