usb: gadget: lpc32xx_udc: fix build error with debugfs enabled

If CONFIG_USB_GADGET_DEBUG_FILES is enabled, lpc32xx_udc breaks
compilation because of a missing include file.

Signed-off-by: Alexandre Pereira da Silva <aletes.xgr@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>