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: Arnd Bergmann <arnd@arndb.de>
Fixes: 308edfdf ("gre6: Cleanup GREv6 receive path, call common GRE functions")
Acked-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>