openvswitch: Fix dependency on IPv6 defrag.

When NF_CONNTRACK is built-in, NF_DEFRAG_IPV6 is a module, and
OPENVSWITCH is built-in, the following build error would occur:

net/built-in.o: In function `ovs_ct_execute':
(.text+0x10f587): undefined reference to `nf_ct_frag6_gather'

Fixes: 7f8a436e ("openvswitch: Add conntrack action")
Reported-by: Jim Davis <jim.epost@gmail.com>
Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>