odhcpd: fix a compilation error

odhcpd: fix a compilation error

odhcpd_get_interface_dns_addr() calls odhcpd_get_interface_linklocal_addr(),
the former takes a const struct interface ptr, the latter takes a non-const
struct interface ptr.

The end result is an unhappy compiler.

Signed-off-by: David Härdeman <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/272
Signed-off-by: Álvaro Fernández Rojas <[email protected]>