[scanner] Create static member for each interface to get static version

[ggaudeau]

I had some instabilities when I run a simple wayland client using waylandpp libraries on several platforms with different desktop environments. I am not sure if it is related but when compiled interface version (such as wl_output) is greater than the one used by the compositor at run-time, I had sometimes an exception that rises up.

This patch does not fix any problem but it allows me to target with more accuracy static and dynamic version currently in use.

Related to this issue: https://github.com/NilsBrause/waylandpp/issues/77