net: ipconfig: move ic_nameservers_fallback into #ifdef block

The new variable is only used when IPCONFIG_BOOTP is defined and otherwise
causes a warning:

net/ipv4/ipconfig.c:177:12: error: 'ic_nameservers_fallback' defined but not used [-Werror=unused-variable]

Move it next to the user.

Fixes: 81ac2722 ("net: ipconfig: Allow DNS to be overwritten by DHCPACK")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>