Add test/lit.site.cfg to .gitignore

This allows to link Polly's lit.site.cfg from the build into the src directory,
without having it removed by every 'git clean':

  ln -s build/tools/polly/test/lit.site.cfg to src/tools/polly/test

Having this file in our src directory allows us to run llvm-lit on specific
test cases in the Polly test directory just by running 'llvm-lit test/case.ll'.