Merge branch 'fix-gitignore' into 'master'

Update gitignore for compatibility with Coq master

See merge request iris/stdpp!103

.gitignore

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