fbdev/sbus: Forward declare all necessary structures in header

In sbuslib.h, declare all of the header file's structs at the
top. No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230806120926.5368-3-tzimmermann@suse.de