Adding *.ptc to github/gitignore/TeX.gitignore

Not sure how to make suggestions for github official code,
but I believe that the titletoc package is sufficiently used
and produces a .ptc metafile for localized Partial Tables of Contents (PTC).

*.ptc is just like the already listed *.toc for the root Table Of Contents (TOC),
and could thus benefit for inclusion in github/gitignore/TeX.gitignore,
listed below: