ipv6: sr: make seg6.h includable without IPv6

include/net/seg6.h cannot be included in a source file if CONFIG_IPV6 is
not enabled:
   include/net/seg6.h: In function 'seg6_pernet':
>> include/net/seg6.h:52:14: error: 'struct net' has no member named
                                        'ipv6'; did you mean 'ipv4'?
     return net->ipv6.seg6_data;
                 ^~~~
                 ipv4

This commit makes seg6_pernet return NULL if IPv6 is not compiled, hence
allowing seg6.h to be included regardless of the configuration.

Signed-off-by: Mathieu Xhonneux <m.xhonneux@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>