Restore makecert script

This was accidentally deleted in https://github.com/OpenXT/openxt/commit/6291173851175e0cfd63986f08ab534cb81e3a75

Previously it was in window/mkbuildmachine but that directory no longer exists and creating it just for this one script may be confusing since we have a tool of the same name. So I've put it in windows/, and will need to update the wiki at https://github.com/OpenXT/openxt/wiki/Windows-Build-Machine to refer to this new path.