Irrlicht 1.8.1 makefile version number fix

The zip-file for Irrlicht 1.8.1 got uploaded once more. As our watchful forum member ent1ty noticed the version number in the Makefile had still been set to 1.8.0. Which resulted in wrong library names on Linux for the shared library. No other changes since last upload. Thanks for the report and sorry for the inconvenience this has caused.