net: ipv6: use ipv6-y directly instead of ipv6-objs

Kbuild supports <modname>-y as well as <modname>-objs.
This simplifies the Makefile.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>