gn/core.gni: add headers

motivation: add more headers to gen/skia.h

Change-Id: I23064ca37afa66123ad00059c1382b906b44cd9e
Reviewed-on: https://skia-review.googlesource.com/c/195362
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
Auto-Submit: Hal Canary <halcanary@google.com>
1 file changed