Remove 'public' from disabled optional target.
Remove 'public' from disabled optional target.

An 'optional' target my wish to define public includes. When the target
is not enabled those public includes should not be present.

Change-Id: I1d0fc4553df100adf9178ad3f57bc0f5bb8f0fcb
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/294658
Reviewed-by: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>
Commit-Queue: Ben Wagner <53965154ffeecc4a10886eab9259e95965f2979a@google.com>

google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia