[json5-parser] Fix build, cleanup

[dg0yt]

No description provided.

[dg0yt]

FTR last upstream change was SEVEN years ago.

Fixes error in https://dev.azure.com/vcpkg/public/_build/results?buildId=107188&view=artifacts&pathAsName=false&type=publishedArtifacts:

CMake Error in CMakeLists.txt:
  No cmake_minimum_required command is present.  A line of code such as

    cmake_minimum_required(VERSION 3.29)

  should be added at the top of the file.  The version specified may be lower
  if you wish to support older CMake versions for this project.  For more
  information run "cmake --help-policy CMP0000".


-- Configuring incomplete, errors occurred!