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>

drivers/usb/gadget/ether.c

@@ -103,7 +103,6 @@ static inline bool has_rndis(void)
  * a "gcc --combine ... part1.c part2.c part3.c ... " build would.
  */
 #include "composite.c"
-#include "usbstring.c"
 #include "config.c"
 #include "epautoconf.c"