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

With minimal changes to keep everything building and functioning.
diff --git a/vulkan.py b/vulkan.py
index 5f46856..95ad639 100755
--- a/vulkan.py
+++ b/vulkan.py
@@ -831,8 +831,12 @@
              Param("const VkRenderPassBegin*", "pRenderPassBegin")]),
 
         Proto("void", "CmdEndRenderPass",
+            [Param("VkCmdBuffer", "cmdBuffer")]),
+
+        Proto("void", "CmdExecuteCommands",
             [Param("VkCmdBuffer", "cmdBuffer"),
-             Param("VkRenderPass", "renderPass")]),
+             Param("uint32_t", "cmdBuffersCount"),
+             Param("const VkCmdBuffer*", "pCmdBuffers")]),
     ],
 )