Only load inputenc when pdftex is used.

Only load inputenc when pdftex is used. Lualatex and XeLaTeX don't need it, and don't like it when that package is loaded. I was helping someone use the Linux Libertine font in his PhD text, and inputenc caused an error with the font specific microtype configuration file. The change corresponds to the following solution: http://tug.org/pipermail/luatex/2010-October/002184.html.