Revert "move vk.xml to src"

This reverts commit 6763d9a73a.

build.zig

@@ -7,7 +7,7 @@ pub fn build(b: *std.Build) void {
     const optimize = b.standardOptimizeOption(.{});
 
     const vk = b.dependency("vulkan-zig", .{
-        .registry = @as([]const u8, b.pathFromRoot("src/vk.xml")),
+        .registry = @as([]const u8, b.pathFromRoot("reg/vk.xml")),
     });
     const vkmod = vk.module("vulkan-zig");