net: Fix build regression in rtl8723bs staging driver.

drivers/staging/rtl8723bs/os_dep/ioctl_cfg80211.c: In function ‘rtw_cfg80211_add_monitor_if’:
drivers/staging/rtl8723bs/os_dep/ioctl_cfg80211.c:2670:10: error: ‘struct net_device’ has no member named ‘destructor’
  mon_ndev->destructor = rtw_ndev_destructor;
          ^

Signed-off-by: David S. Miller <davem@davemloft.net>