usb: gadget: start with libcomposite

This patch aims to be simple. It removes #include usbstribgs.c line from each
gadget and creates libcomposite.ko which has only one member, that is
usbstribgs.c.

Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>