Update gitignore

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).

.gitignore

 *.vo
+*.vos
+*.vok
 *.vio
 *.v.d
 .coqdeps.d
+.Makefile.coq.d
 *.glob
 *.cache
 *.aux