flake: Fix SVG symbol elements being rendered outside of use

The <symbol> element shall not be rendered directly, only rendered when referenced by a <use> element.

Test Plan

1. Take BalloonSymbols.svg and move all symbols outside of the <defs> element (or comment out the start and end <defs> tags)
2. Open the file in Krita
3. Check that overlapping black copies of the speech balloon symbols does not exist on the top left corner and the blue copies are nicely arranged on the canvas.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.

Reminder: the reviewer is responsible for merging the patch, this is to ensure at the least two people can build the patch. In case a patch breaks the build, both the author and the reviewer should be contacted to fix the build. If this is not possible, the commits shall be reverted, and a notification with the reasoning and any relevant logs shall be sent to the mailing list, kimageshop@kde.org.