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>
diff --git a/BUILD.gn b/BUILD.gn
index 3644a8a..99d0d57 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -743,7 +743,7 @@
 
   public_defines = [ "SK_USE_SKCMS" ]
   deps = [
-    "//third_party/skcms",
+    "third_party/skcms",
   ]
   sources = [
     "src/core/SkColorSpaceXform_skcms.cpp",