Add skcms directory to public includes

skcms is part of Skia's public API now. This attempts to recognize that,
and pave the way for moving the header to another location more easily
in a follow up CL, or - at a minimum - for clients that redistribute
Skia as a library + includes to relocate the skcms.h header as part of
that.

Change-Id: I15da63b0d4ab8916a71fb7e6ab3656db87252707
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/209640
Reviewed-by: Mike Klein <mtklein@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
14 files changed