Allow <vboot/crossystem.h> to be usable in C++ code. BUG=None TEST=build update_engine with a change that uses the header. BRANCH=none Change-Id: Icbfe9be615a4f7f4078a0a0cde64324908dea2a7 Reviewed-on: https://chromium-review.googlesource.com/174428 Commit-Queue: Richard Barnette <[email protected]> Tested-by: Richard Barnette <[email protected]> Reviewed-by: Bill Richardson <[email protected]>