rt2x00: Fix building of rt2800lib when rt2x00 driver is built-in.

When enabling rt2800usb as a built-in driver (as opposed to a as a module) the build fails. See
http://marc.info/?l=linux-wireless&m=125768687711034&w=2 for details.

Fix it by properly including <linux/usb.h> from rt2x00usb.h
Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/rt2x00/rt2x00usb.h

@@ -26,6 +26,8 @@
 #ifndef RT2X00USB_H
 #define RT2X00USB_H
 
+#include <linux/usb.h>
+
 #define to_usb_device_intf(d) \
 ({ \
 	struct usb_interface *intf = to_usb_interface(d); \