update vulkan.h for multi-level command buffers (V121)

With minimal changes to keep everything building and functioning.
diff --git a/include/vkLayer.h b/include/vkLayer.h
index a1104db..6467354 100644
--- a/include/vkLayer.h
+++ b/include/vkLayer.h
@@ -122,6 +122,7 @@
     PFN_vkCreateRenderPass CreateRenderPass;
     PFN_vkCmdBeginRenderPass CmdBeginRenderPass;
     PFN_vkCmdEndRenderPass CmdEndRenderPass;
+    PFN_vkCmdExecuteCommands CmdExecuteCommands;
     PFN_vkCreateSwapChainWSI CreateSwapChainWSI;
     PFN_vkDestroySwapChainWSI DestroySwapChainWSI;
     PFN_vkGetSwapChainInfoWSI GetSwapChainInfoWSI;