ip_gre: Fix dependencies wrt. ipv6.

The GRE tunnel driver needs to invoke icmpv6 helpers in the
ipv6 stack when ipv6 support is enabled.

Therefore if IPV6 is enabled, we have to enforce that GRE's
enabling (modular or static) matches that of ipv6.
Reported-by: NPatrick McHardy <kaber@trash.net>
Reported-by: NHerbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/Kconfig

@@ -217,6 +217,7 @@ config NET_IPIP
 
 config NET_IPGRE
 	tristate "IP: GRE tunnels over IP"
+	depends on IPV6 || IPV6=n
 	help
 	  Tunneling means encapsulating data of one protocol type within
 	  another protocol and sending it over a channel that understands the