Use relative path to skcms GN file

This allows clients to find this dependency when skia isn't the root of
the build.

Bug: skia:
Change-Id: Ic1b075f962ea754fd02acc17f44363193a2bf9d1
Reviewed-on: https://skia-review.googlesource.com/123043
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
Auto-Submit: Brian Osman <brianosman@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
1 file changed