Rename c++-mode files to work on Windows.

Previously, if you tried to to clone this git repository you receive the
following error message:

error: unable to create file c++-mode/<sstream> (Invalid argument)
error: unable to create file c++-mode/d_operator<< (Invalid argument)
error: unable to create file c++-mode/operator<< (Invalid argument)
error: unable to create file c++-mode/operator>> (Invalid argument)
error: unable to create file c++-mode/std:: (Invalid argument)
fatal: unable to checkout working tree
warning: Clone succeeded, but checkout failed.
You can inspect what was checked out with 'git status'
and retry the checkout with 'git checkout -f HEAD'