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: 7f8a436eaa2c ("openvswitch: Add conntrack action")
Reported-by: Jim Davis <[e-mail]>
Signed-off-by: Joe Stringer <[e-mail]>
Acked-by: Pravin B Shelar <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/openvswitch/Kconfig

···
 config OPENVSWITCH
 	tristate "Open vSwitch"
 	depends on INET
-	depends on (!NF_CONNTRACK || NF_CONNTRACK)
+	depends on !NF_CONNTRACK || \
+		   (NF_CONNTRACK && (!NF_DEFRAG_IPV6 || NF_DEFRAG_IPV6))
 	select LIBCRC32C
 	select MPLS
 	select NET_MPLS_GSO