idr: export idr_get_next()

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: Ben Hutchings <[e-mail]>
Acked-by: KAMEZAWA Hiroyuki <[e-mail]>
Signed-off-by: Artem Bityutskiy <[e-mail]>
Signed-off-by: David Woodhouse <[e-mail]>

lib/idr.c

···
 	}
 	return NULL;
 }
-
+EXPORT_SYMBOL(idr_get_next);
 
 
 /**