Fix some gcc 7.3 reported warnings

Fix some gcc 7.3 reported warnings

```
src/client/joystick_controller.h:85:2: warning: duplicate 'virtual' declaration specifier [-Wduplicate-decl-specifier]
        virtual bool isTriggered(const irr::SEvent::SJoystickEvent &ev) const;
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/7.3.1/../../../../include/c++/7.3.1/ext/new_allocator.h:140:22: warning: destructor called on non-final 'JoystickAxisCmb' that has virtual functions but non-virtual destructor [-Wdelete-non-virtual-dtor]
                destroy(_Up* __p) { __p->~_Up(); }
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/7.3.1/../../../../include/c++/7.3.1/ext/new_allocator.h:140:22: warning: destructor called on non-final 'JoystickButtonCmb' that has virtual functions but non-virtual destructor [-Wdelete-non-virtual-dtor]
        destroy(_Up* __p) { __p->~_Up(); }
```