Don't install cpp_bindgen via user using FetchContent

[havogt]

cpp_bindgen does not compile anything, but just installs sources which are then (automatically) compiled by the user.

In the case where the user uses FetchContent, the install target of cpp_bindgen will be called, which installs the sources of cpp_bindgen to the location defined by the user-CMake. This doesn't harm but might be confusing.