usb: cdc-wdm: export cdc-wdm uapi header

The include/uapi/linux/usb/cdc-wdm.h header defines cdc-wdm
userspace APIs and should be exported by make headers_install.

Cc: <stable@vger.kernel.org> # 3.10, 3.12, 3.14
Fixes: 3edce1cf ("USB: cdc-wdm: implement IOCTL_WDM_MAX_COMMAND")
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>