fix failing build

[mildsunrise]

Description

current code in master fails to build because Json.cpp uses std::filesystem without including <filesystem>

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• [X] I have read the Contributing Guidelines.
• [X] All commit messages are properly formatted and commits squashed where appropriate.
• [X] My code is not on master or a release/* branch.
• [X] The code has been tested.

[tt2468]

On what environment is build failing for you?

[mildsunrise]

it's been half a year. I don't know the exact environment that this failure happened on, but this mistake seemed trivial enough that i didn't debug back then. If it helps I was using Arch Linux, and building obs-studio from latest master at that time. I'll try to reproduce...