private: Remove unnecessary forward declarations

Declarations for wl_connection and wl_closure are not needed here.
wl_closure already has a complete definition.
Removing these forward declarations results in a clean, warning-free compile.

Signed-off-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>
[Updated to apply to trunk]
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>