USB: gadget: export functionfs.h to the includes available for userspace

To compile functionfs userspace driver one needs definitions from
include/linux/usb/functionfs.h. This patch add this file to the list
of includes exported to userspace.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>