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>

include/linux/usb/Kbuild

@@ -2,6 +2,7 @@ header-y += audio.h
 header-y += cdc.h
 header-y += ch9.h
 header-y += ch11.h
+header-y += functionfs.h
 header-y += gadgetfs.h
 header-y += midi.h
 header-y += g_printer.h