Update gitignore for compatibility with Coq master

See https://github.com/coq/coq/pull/10947 (.coqdeps.d now uses the name of the Coq Makefile) and https://github.com/coq/coq/pull/8642 (Coq now generates empty interface files *.vos when compiling).

Thanks a lot!

mentioned in commit af7e8036

merged