Add third_party/khronos to GN configs in ozone gbm

Add third_party/khronos to GN configs in ozone gbm

The khronos headers are present in GYP, but missing in GN, causing
the build using GN to fail when 'ozone_platform_gbm=true' is used.

BUG=487108
Committed: https://crrev.com/a16802db6f81062df2fb00d6a7d7f5c04f4dd20c
Cr-Commit-Position: refs/heads/master@{#329411}

[spang, 2015-05-12 15:05:09]

spang@chromium.org changed reviewers:
+ spang@chromium.org

[spang, 2015-05-12 15:05:09]

The CQ bit was checked by spang@chromium.org

[spang, 2015-05-12 15:05:09]

lgtm

[commit-bot: I haz the power, 2015-05-12 15:05:32]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1129713003/1

[commit-bot: I haz the power, 2015-05-12 15:54:13]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-12 15:54:53]

Patchset 1 (id:??) landed as
https://crrev.com/a16802db6f81062df2fb00d6a7d7f5c04f4dd20c
Cr-Commit-Position: refs/heads/master@{#329411}