ufbx: install extra files to includedir (#6233)

Install the files under the "extra" folder for convenient use in C++ project and others.

packages/u/ufbx/xmake.lua

@@ -28,6 +28,7 @@ package("ufbx")
                 end
         ]])
         import("package.tools.xmake").install(package)
+        os.trycp("extra/*", package:installdir("include"))
     end)
 
     on_test(function (package)