cmake: Require nlohmann-json when building the front-end

[rmnvgr]

Explain the Pull Request

Nlohmann-json is used in the about dialog, yet wasn't required when building the front-end, leading to failing compilation. This ensures that it is correctly required. See flathub/flathub#3159 (comment).

Completion Checklist

• I have added myself to the Copyright and License headers and files.
• I will maintain this code in the future and have added myself to CODEOWNERS. <-- I haven't added myself to CODEOWNERS, do I need to for such a simple patch?
• I have tested this change on the following platforms:
  • MacOS 10.15
  • MacOS 11
  • MacOS 12
  • Ubuntu 20.04
  • Ubuntu 22.04
  • Windows 10
  • Windows 11

[Xaymar]

LGTM. Will squash to remove the manual new lines in the message though, unless you want to fix those yourself?

I haven't added myself to CODEOWNERS, do I need to for such a simple patch?

Nope, it's perfectly fine to submit patches without adding yourself to coode owners. This is more of a requirement for feature additions

[rmnvgr]

I have removed the manual new lines.

[Xaymar]

May have actually squashed instead of rebasing as intended. Oh well.