net: ipconfig: avoid warning by making ic_addrservaddr static

The symbol ic_addrservaddr is not static, but has no declaration
to match so make it static to fix the following warning:

net/ipv4/ipconfig.c:130:8: warning: symbol 'ic_addrservaddr' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>