Make WiFiManagerParameter destructor virtual

[rguca]

If a instance of WiFiManagerParameter is created by new and deleted by delete, a compiler warning is issued.

src/settings/SettingsController.cpp:78:67: warning: deleting object of polymorphic class type 'WiFiManagerParameter' which has non-virtual destructor might cause undefined behavior [-Wdelete-non-virtual-dtor]
   78 |   for (WiFiManagerParameter* p : this->wifi_settings->parameters) delete p;

The destructor should be virtual, because the class can be extended.