kernel-interface: Add destination prefix to get_nexthop()

This allows to determine the next hop to reach a subnet, for instance, when
installing routes for shunt policies.

src/libcharon/plugins/kernel_iph/kernel_iph_net.c

@@ -562,7 +562,7 @@ METHOD(kernel_net_t, get_source_addr, host_t*,
 }
 
 METHOD(kernel_net_t, get_nexthop, host_t*,
-	private_kernel_iph_net_t *this, host_t *dest, host_t *src)
+	private_kernel_iph_net_t *this, host_t *dest, int prefix, host_t *src)
 {
 	MIB_IPFORWARD_ROW2 route;
 	SOCKADDR_INET best, *sai_dst, *sai_src = NULL;