usb: fix implicit usage of gfp.h in host/xhci-hub.c

To fix this build error on ARM:

drivers/usb/host/xhci-hub.c: In function 'xhci_stop_device':
drivers/usb/host/xhci-hub.c:261: error: 'GFP_NOIO' undeclared (first use in this function)
make[4]: *** [drivers/usb/host/xhci-hub.o] Error 1
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>

drivers/usb/host/xhci-hub.c

@@ -20,6 +20,7 @@
  * Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
  */
 
+#include <linux/gfp.h>
 #include <asm/unaligned.h>
 
 #include "xhci.h"