scanner: Remove unused forward decs from client protocol

wayland-client-protocol.h had forward declarations for wl_client and
wl_resource, yet nothing on the client side references these types.

Add a 'side' condition to only generate these forward declarations in the
server protocol header.

Signed-off-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by:  <pekka.paalanen@collabora.co.uk>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>