[svg] Add public.bzl defs for SVG source sets

Change-Id: I2c690b1e079494a3e1217330c3791d29bc37e27c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/325337
Commit-Queue: Florin Malita <fmalita@google.com>
Reviewed-by: Tyler Denniston <tdenniston@google.com>
1 file changed