apps/nshlib/: In netcommds.c, the UNUSED() macro may be applied to an undefined symbol when IPv6 is enabled but IPv4 is not.

nshlib/nsh_netcmds.c

@@ -947,6 +947,8 @@ int cmd_ifconfig(FAR struct nsh_vtbl_s *vtbl, int argc, char **argv)
   UNUSED(hostip);
   UNUSED(mask);
   UNUSED(gwip);
+#endif
+#ifdef CONFIG_NET_IPv4
   UNUSED(gip);
 #endif