idr_get_next() was accidentally not exported when added.  It is about
to be used by mtdcore, which may be built as a module.
Signed-off-by: NBen Hutchings <bhutchings@solarflare.com>
Acked-by: NKAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: NArtem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: NDavid Woodhouse <David.Woodhouse@intel.com>