vulkan: Add function stub & plumbing for KHR_swapchain_front_buffered

Test: build
Change-Id: I6ea2e33e708067f11069bda19c4b5ea167158913
diff --git a/vulkan/libvulkan/driver_gen.h b/vulkan/libvulkan/driver_gen.h
index fa81958..a137ab6 100644
--- a/vulkan/libvulkan/driver_gen.h
+++ b/vulkan/libvulkan/driver_gen.h
@@ -40,6 +40,7 @@
         KHR_surface,
         KHR_swapchain,
         GOOGLE_display_timing,
+        KHR_swapchain_front_buffered,
 
         EXTENSION_CORE,  // valid bit
         EXTENSION_COUNT,