[skshaper] Export "primitive" sources in public.bzl

The harfbuzz backend is not usable in G3 at this point (missing
SkLoadICU header).  Making the primitive backend available to unblock
Skottie progress.

TBR=
Bug: skia:8696
Change-Id: I5aa1003f33926450f6c7512736cd71f9e1c80297
Reviewed-on: https://skia-review.googlesource.com/c/185600
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
1 file changed