Add "rt" to libs of GN build linux sandbox unittests.

Add "rt" to libs of GN build linux sandbox unittests.

This is causing a compile failure in component mode.

TBR=dpranke

Committed: https://crrev.com/e3934435b6ad1e97910e532e490b8d672d34ffba
Cr-Commit-Position: refs/heads/master@{#294658}

[brettw, 2014-09-12 21:07:13]

brettw@chromium.org changed reviewers:
+ dpranke@chromium.org

[Dirk Pranke, 2014-09-12 21:09:18]

lgtm

[brettw, 2014-09-12 21:15:13]

Committed patchset #1 (id:1) to pending queue manually as c03e515 (presubmit
successful).

[commit-bot: I haz the power, 2014-09-12 21:20:53]

Patchset 1 (id:??) landed as
https://crrev.com/e3934435b6ad1e97910e532e490b8d672d34ffba
Cr-Commit-Position: refs/heads/master@{#294658}