Add back submodule for less v1.3.3.

This is needed to get dijit/themes/claro/compile.js and dojox/trunk/mobile/themes/utils/compile.sh
working again.  Might be possible to install less via "npm install" instead but this is the path of
least resistance.  Less submodule was originally part of 8525a1a318033ea75bef49c54c1e91688aad49e9.
(cherry picked from commit 174c5bd165c09036a731b771fe474fd104044ecc)