Fix cmake installation

[Kaedras]

Importing this library in cmake using find_package(ValveFileVDF REQUIRED) fails with the following error:

CMake Error at /usr/cmake/ValveFileVDFConfig.cmake:27 (include):
  include could not find requested file:

    /usr/cmake/ValveFileVDFTargets.cmake

This pull request fixes the issue and also fixes the installation path of the cmake files so they get installed into lib64/cmake/ValveFileVDF instead of cmake.

With these changes find_package(ValveFileVDF) and target_link_libraries(<target> PRIVATE ValveFileVDF::ValveFileVDF) work as expected for both system-wide installation and when using CMAKE_INSTALL_PREFIX / CMAKE_PREFIX_DIR.

[TinyTinni]

Thank you very much.