Fix translator having multiply defined symbols in GN builds. This error was introduced in 82b5ab606f68bad27fe5d143d8b96e84801222ac 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/321770 Reviewed-by: Kenneth Russell <kbr@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Corentin Wallez <cwallez@chromium.org>