Enqueue stylesheet in plugin for wp-login.php

You can do this two way.But I prefer to go for the second way.You need to put this code in your plugins file.

First way:

function my_loginlcustomization() {
  echo '<style type="text/css">
    h1 a {
     background-image: url(' . plugin_dir_url( __FILE__ ).'/login/logo.png) !important;
    }
  </style>';
}
add_action('login_head', 'my_loginlcustomization');

Second Way(I prefer):

function my_loginlcustomization() {

wp_register_style('custom_loginstyle', plugins_url('/css/login.css', __FILE__));
wp_enqueue_style("custom_loginstyle");
}
add_action('login_head', 'my_loginlcustomization'); 

Thanks