cros_ec: add chrome EC headers to include path

cros_ec: add chrome EC headers to include path

This adds the path to chrome EC headers to the depthcharge
build. Depthcharge currently includes a manually maintained copy of
the EC headers which are perpetually out of sync with the real
interface definitions. By adding the include path, we can build
depthcharge with the actual EC interface definitions and eliminate the
manual maintenance of copies of EC headers. Once the include path is
in place, we can remove the copies of the EC headers from depthcharge.

BUG=b:152373049

Change-Id: I1ce0ad9dc99ea52f177d4fb034fd23efd95a7864
Signed-off-by: Caveh Jalali <caveh@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/39947
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Duncan Laurie <dlaurie@chromium.org>