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>

drivers/net/wireless/b43/leds.h

 #ifndef B43_LEDS_H_
 #define B43_LEDS_H_
 
+struct b43_wl;
 struct b43_wldev;
 
 #ifdef CONFIG_B43_LEDS