usb: gadget: configfs: include appropriate header file in configfs.c

Include appropriate header file drivers/usb/gadget/configfs.h in
gadget/configfs.c because function unregister_gadget_item() has its
prototype declaration in gadget/configfs.h.

This eliminates the following warning in gadget/configfs.c:
drivers/usb/gadget/configfs.c:994:6: warning: no previous prototype for ‘unregister_gadget_item’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>