pppd: Fix compilation on Linux when IPV6 is disabled (#360)
This rearranges the PPP_WITH_IPV6CP guards added in commit
80b8744eb42c ("Changing INET6 to PPP_WITH_IPV6CP and adding configure
option", 2021-08-06) so that we (a) always include the rtnetlink
headers, since we need them for get_ppp_stats_rtnetlink(), and (b)
don't include eui64.h unless we have IPV6 support.
Fixes: 80b8744eb42c ("Changing INET6 to PPP_WITH_IPV6CP and adding configure option")
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>