Steel magnate Lakshmi Mittal, the major shareholder in steelmaker ArcelorMittal, is considering entering the takeover battle for Anglo-Australian mining giant Rio Tinto, according to the Financial Times.    Citing unnamed individuals familiar with the matter, the FT said that Mittal was interested in securing larger supplies of iron ore.   "Mr Mittal has considered some involvement in the takeover, such as the idea of taking a stake in Rio through buying from existing shareholders," an unnamed banker was quoted as saying.   "On the other hand, he could wait until l