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>