Move install rules under PROJECT_IS_TOP_LEVEL

[Shatur]

What does this change do? I moved install rules under PROJECT_IS_TOP_LEVEL check. Otherwise using FetchContent it will generate install rule for this library which is unnecessary since the library is already added to the project.

Pre-merge checklist

• [x] I've read CONTRIBUTING.md
• [x] I've rebased my changes against the current HEAD of origin/master (if necessary)
• [ ] I've added new test cases to verify my change (unnecessary)
• [ ] I've regenerated toml.hpp (how-to) (unnecessary)
• [ ] I've updated any affected documentation (unnecessary)
• [ ] I've rebuilt and run the tests with at least one of: (unnecessary)
  • [ ] Clang 6 or higher
  • [ ] GCC 7 or higher
  • [ ] MSVC 19.20 (Visual Studio 2019) or higher
• [ ] I've added my name to the list of contributors in README.md (unnecessary, it's a super minor change)

[marzer]

Sounds reasonable to me, thanks!