ipv6: Make ip6_route_output_flags_noref() static.

This function is only used in net/ipv6/route.c and has no reason to be
visible outside of it.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/50706db7f675e40b3594d62011d9363dce32b92e.1674495822.git.gnault@redhat.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>