CraftCMS > UserSessionService should be compatible with CWebUser
It's a known issue you can safely ignore on Craft 2, that's been resolved for Craft 3.

PHP 7.0 (or maybe it was 7.1) changed the way that particular error was reported from E_STRICT (which Craft 2 is suppressing) to E_WARNING, which Craft 2 isn't suppressing, so now it pops up on latter versions of PHP.