selftests/bpf: bring back (void *) cast to set_ipv4_csum in test_tc_tunnel

It was removed in commit 166b5a7f ("selftests_bpf: extend
test_tc_tunnel for UDP encap") without any explanation.

Otherwise I see:
progs/test_tc_tunnel.c:160:17: warning: taking address of packed member 'ip' of class or structure
      'v4hdr' may result in an unaligned pointer value [-Waddress-of-packed-member]
        set_ipv4_csum(&h_outer.ip);
                       ^~~~~~~~~~
1 warning generated.

Cc: Alan Maguire <alan.maguire@oracle.com>
Cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Fixes: 166b5a7f ("selftests_bpf: extend test_tc_tunnel for UDP encap")
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Acked-by: Song Liu <songliubraving@fb.com>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>