[ticket/11398] Correctly call permission_set method in permission tool

The permission_set method calls itself inside the permission tool.
Probably due to an oversight, it is called as $this->set(), which causes a
fatal error. This patch will get rid of this issue.

PHPBB3-11398
Merge remote-tracking branch 'remotes/marc/ticket/11398' into develop

# By Marc Alexander
# Via Marc Alexander
* remotes/marc/ticket/11398:
  [ticket/11398] Correctly call permission_set method in permission tool