in.h: Coordinate in6_pktinfo and ip6_mtuinfo for kernel and glibc [BZ #15850]

Similarly to what we did for in6_addr, we need a macro
to guard in6_pktinfo and ip6_mtuinfo too.

Cc: Carlos O'Donell <carlos@redhat.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>