This error was introduced in 82b5ab60
where the translator's essl and glsl sources are compiled in both
the translator and translator_lib targets. As translator depends on
translator_lib this caused symbols to be defined multiple times.

BUG=angleproject:1250

Change-Id: I50bab7112d1460e14710f50250fa2417b2163236
Reviewed-on: https://chromium-review.googlesource.com/321770Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>