cursor: add forward declaration for struct wl_buffer

This makes the header self-contained, since the struct is considered
opaque from waylad-cursor POV.

As we're here move the wl_shm fwd. declaration alongside the others.
Making it easier to read and track.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>