b43: add 'struct b43_wl' missing declaration

'struct b43_wl' declaration is missing at 'leds.h'.
It should be declared to avoid getting some GCC warnings at 'b43_leds_unregister'.

Signed-off-by: Miguel Botón <mboton@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>