Adding a copyright SPDX back to the mesh primitive restart extension (KhronosGroup#2533)

Author: weegeekps

extensions/2.0/Vendor/EXT_mesh_primitive_restart/README.md

@@ -1,3 +1,8 @@
+<!--
+Copyright 2025 Bentley Systems, Incorporated
+SPDX-License-Identifier: CC-BY-4.0
+-->
+
 # EXT_mesh_primitive_restart
 
 ## Contributors