Add missing headers to sksl.gni.

Change-Id: I225de583d408f1ca29e7e74d591e9be1ab2cbc20
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/532785
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
1 file changed