[Impeller] Enable X11 and Wayland support for Vulkan (flutter#639)

Author: iskakaushik

build_overrides/vulkan_headers.gni

@@ -4,3 +4,8 @@
 
 # This file is needed by the vulkan-headers build, but doesn't need to actually
 # set anything.
+
+if (is_linux) {
+  vulkan_use_x11 = true
+  vulkan_use_wayland = true
+}