scanner: don't emit the extern declarations for external types

We were emitting the extern declarations of all types used in the protocol,
even if not defined in it. This caused warnings to be produced when using
the -Wredundant-decls compiler flag when building an extension that uses
e.g. wl_surface. However we only need the extern declarations if the
protocol defines a factory for those external interfaces. That is a
bad design and can be however done by including the dependent protocol
header first.
So only emit the extern declarations for the types that the protocol
actually defines, this restoring the behavior we were using in 1.7.

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=90677



Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Tested-by: Arnaud Vrac <rawoul@gmail.com>