Only deqp uses librt so the dependency belongs there, not in util.gyp,
and the condition should be OS==linux, not use_x11.
This lets things work on Chrome OS.

BUG=angleproject:1297

Change-Id: If397da5463b9c2d5217eb3b20841fa5a44bf56a3
Reviewed-on: https://chromium-review.googlesource.com/334813Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>