WAF: Fix detection of IPv6

WAF: Fix detection of IPv6

Detection of IPv6 failed with strict CFLAGS due to missing
header file.

  Checking for HAVE_IPV6       : not found

../test.c: In function ‘main’:
../test.c:226:34: error: implicit declaration of function
    ‘if_nametoindex’ [-Werror=implicit-function-declaration]
                        int idx = if_nametoindex("iface1");
                                  ^~~~~~~~~~~~~~

Signed-off-by: Lukas Slebodnik <lslebodn@redhat.com>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>
Autobuild-User(master): Ralph Böhme <slow@samba.org>
Autobuild-Date(master): Mon Jan  2 18:03:20 CET 2017 on sn-devel-144