Update Coq.gitignore after coq/coq#11075 (#3247)

Add .vok files from https://github.com/coq/coq/pull/11075

Coq.gitignore

@@ -18,6 +18,7 @@
 *.v.d
 *.vio
 *.vo
+*.vok
 *.vos
 .coq-native/
 .csdp.cache