Add SkVx.h to core.gni

Trying to run Chrome trybots on changes to SkVx.h,
but it's deciding to skip the build...

Change-Id: Ic40cc2eec5db00c4f6eacc72e0e50101a8478e99
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/317596
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
1 file changed