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

With minimal changes to keep everything building and functioning.
diff --git a/layers/draw_state.h b/layers/draw_state.h
index 6f8f016..c46a28a 100644
--- a/layers/draw_state.h
+++ b/layers/draw_state.h
@@ -211,6 +211,7 @@
     CMD_SAVEATOMICCOUNTERS,
     CMD_BEGINRENDERPASS,
     CMD_ENDRENDERPASS,
+    CMD_EXECUTECOMMANDS,
     CMD_DBGMARKERBEGIN,
     CMD_DBGMARKEREND,
 } CMD_TYPE;