net/if: remove the macro expansion of the public structure

This is a compatibility issue if different structures included
in the usrsock server/client scene, usrsock client has no habit
of actively choosing of config CONFIG_NETDEV_IFINDEX at most of the time

Change-Id: Ib18b0ee12c76de759f6e8a54375c3adcdc31353a
Signed-off-by: chao.an <anchao@xiaomi.com>

include/net/if.h

@@ -209,9 +209,7 @@ struct lifconf
 struct ifreq
 {
   char                        ifr_name[IFNAMSIZ];       /* Network device name (e.g. "eth0") */
-#ifdef CONFIG_NETDEV_IFINDEX
   int16_t                     ifr_ifindex;              /* Interface index */
-#endif
   union
   {
     struct sockaddr           ifru_addr;                /* IP Address */