gre6: add Kconfig dependency for NET_IPGRE_DEMUX

The ipv6 gre implementation was cleaned up to share more code
with the ipv4 version, but it can be enabled even when NET_IPGRE_DEMUX
is disabled, resulting in a link error:

net/built-in.o: In function `gre_rcv':
:(.text+0x17f5d0): undefined reference to `gre_parse_header'
ERROR: "gre_parse_header" [net/ipv6/ip6_gre.ko] undefined!

This adds a Kconfig dependency to prevent that now invalid
configuration.
Signed-off-by: NArnd Bergmann <arnd@arndb.de>
Fixes: 308edfdf ("gre6: Cleanup GREv6 receive path, call common GRE functions")
Acked-by: NTom Herbert <tom@herbertland.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/Kconfig

@@ -218,6 +218,7 @@ config IPV6_GRE
 	tristate "IPv6: GRE tunnel"
 	select IPV6_TUNNEL
 	select NET_IP_TUNNEL
+	depends on NET_IPGRE_DEMUX
 	---help---
 	  Tunneling means encapsulating data of one protocol type within
 	  another protocol and sending it over a channel that understands the