ipv6: Make __ipv6_select_ident static

Make __ipv6_select_ident() static as it isn't used outside
the file.

Fixes: 0508c07f (ipv6: Select fragment id during UFO segmentation if not set.)
Signed-off-by: Vladislav Yasevich <vyasevic@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>