Fix startup sound for Planck (#8852)

[JunoNgx]

Description

Add to Planck config to resolve #8852, as suggested by Drashna.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout (addition or update)
• Documentation

Issues Fixed or Closed by This PR

• Issue [Bug] Audio on Planck since last breaking changes 2020/02 #8825, where Planck startup sound doesn't play.

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[drashna]

As audio is enabled at the keyboard level, it would probably be best to enable this at the keyboard level too. keyboards/planck/config.h already has some audio config there, so setting it there should be okay-ish. (and this setting doesn't really hurt the boards that don't need it...)

[JunoNgx]

Config moved to keyboards/planck/config.h as suggested by @drashna. Let me know if anything else should be done.