Callouts icons are not shown in PDF export

[rednaxLamington]

Describe the bug When exporting a page to pdf containing callouts, the icons are not shown.

Steps To Reproduce Steps to reproduce the behavior:

1. Create a page like this:

2. Export the page to PDF

3. Open the PDF document

4. See the icons are missing:

Expected behavior The PDF should also show the icons.

Your Configuration (please complete the following information):

• Exact BookStack Version (Found in settings): v21.04.5
• PHP Version: 7.4.16
• Hosting Method (Nginx/Apache/Docker): Windows 10 / IIS

Additional context Add any other context about the problem here.