Add missing GN build dependency on -lrt for linux. Without this fix, angle_end2end_tests links fine on Trusty but not on Precise or older versions of Linux (util/linux/LinuxTimer.cpp calls clock_gettime(), which is part of libc on newer versions of the OS but requires -lrt on older versions. GYP had the dependency specified but GN didn't). R=cwallez@chromium.org BUG=432959, 531296 Change-Id: I70e1f701d099f3ba72c7e5dfd7af6d47658ffad9 Reviewed-on: https://chromium-review.googlesource.com/312298 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Tested-by: Corentin Wallez <cwallez@chromium.org>