Fix Vulkan-only build The translator_gl_d3d_only target cannot build if neither of the gl or d3d backends are built. This target is used by Chrome's validating command decoder only. Bug: angleproject:5922 Change-Id: Ibebae6d6aa78ad88498a727d3b0e1695bb110535 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2871969 Reviewed-by: Tim Van Patten <timvp@google.com> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>