lib/Kconfig: fix OBJAGG in lib/ menu structure

Keep the "Library routines" menu intact by moving OBJAGG into it.
Otherwise OBJAGG is displayed/presented as an orphan in the
various config menus.

Fixes: 0a020d41

 ("lib: introduce initial implementation of object aggregation manager")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jiri Pirko <jiri@mellanox.com>
Cc: Ido Schimmel <idosch@mellanox.com>
Cc: David S. Miller <davem@davemloft.net>
Tested-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>