fix this warning:

  net/ax25/sysctl_net_ax25.c:27: warning: ‘min_ds_timeout’ defined but not used
  net/ax25/sysctl_net_ax25.c:27: warning: ‘max_ds_timeout’ defined but not used

These are only used in the CONFIG_AX25_DAMA_SLAVE case.
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: David S. Miller <davem@davemloft.net>