Seedlet: Fix a fatal error in Gutenberg (#7940)

4bf4fb9a introduced a bug when including `wpcom-colors.php` by using the
editor. It was missing a directory separate and so  it fataled with an
error message like:
`PHP Fatal error:  Uncaught Error: Failed opening required '/xxx/seedlet/incwpcom-colors.php' (include_path='.') in /xxx/seedlet/inc/wpcom-editor-colors.php:3

seedlet/inc/wpcom-editor-colors.php

@@ -1,3 +1,3 @@
 <?php declare( strict_types = 1 ); ?>
 <?php
-require_once __DIR__ . 'wpcom-colors.php';
+require_once __DIR__ . '/wpcom-colors.php';