platform: Fix build with LIBNL_INET6_ADDR_GEN_MODE

  platform/nm-linux-platform.c: In function 'setup':
  platform/nm-linux-platform.c:4364:2: error: 'object' undeclared (first use in this function)
    object = nl_cache_get_first (priv->link_cache);
    ^

Fixes 2b8060b9b36fe8f540b38c085600f3771d4ac6b4