Hi,
You should place it before the last ?> you can move that down a few lines, it is the end of the file.
Your function should look like this:

<?php 

// logo personnalise
function childtheme_custom_login() {
echo '<link rel="stylesheet" type="text/css" href="' . get_bloginfo('stylesheet_directory') . '/login.css" />';
}

add_action('login_head', 'childtheme_custom_login');

function add_google_remarketing(){
?>
<script type="text/javascript">
var districtmPixel = {};
districtmPixel.revenueValue = '0.00';
(function(){
var dm = document.createElement('script');
dm.async = true;
dm.src = '//cdn.pixlads.com/agencies/1/advertisers/1163/audience.js';
var ref = document.getElementsByTagName('script')[0];
ref.parentNode.insertBefore(dm, ref);
})();
</script>
<?php
}
add_action('wp_footer', 'add_google_remarketing');


?>

Please make these changes via FTP or your webhost file manager, so if you make a mistake and your site crashes you can edit the file back to the way it was.
Notice how I placed your code with the