Vulkan: Fix gcc compile error Add 'no-unused-function' to compile glslang BUG=angleproject:2301 Change-Id: Iac741e007873fb718527514210649e6b658a63b8 Reviewed-on: https://chromium-review.googlesource.com/898394 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>