bazel: refactor SkSL private headers into own filegroup.

Files in :private_hdrs map to gn/core.gni and :sksl_private_hdrs
maps to gn/sksl.gni.

Bug: skia:13485
Change-Id: I9f97d88b886ac53faffbf9ca51d743b543785aa1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/568039
Reviewed-by: Kevin Lubick <kjlubick@google.com>
1 file changed