net: ipv6: ndisc: Fix warning when CONFIG_SYSCTL=n

When CONFIG_SYSCTL=n the following build warning happens:

net/ipv6/ndisc.c:1730:1: warning: label 'out' defined but not used [-Wunused-label]

The 'out' label is only used when CONFIG_SYSCTL=y, so move it inside the
'ifdef CONFIG_SYSCTL' block.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>