QWaylandInputDevice: Fix missing virtual destructor declaration

QWaylandInputDevice recently gained a virtual function, making it a
polymorphic class. It should thus have a virtual destructor as well.

Discovered through a GCC diagnostic:

warning: deleting object of polymorphic class type 'QWaylandInputDevice' which has non-virtual destructor might cause undefined behaviour [-Wdelete-non-virtual-dtor]

Change-Id: Ie816caabea56cde320768dcc8aba6b5ad50bf4c1
Reviewed-by: Giulio Camuffo <giulio.camuffo@jollamobile.com>