Move vtk_module_third_party() call back to the top

The rest of the file depends on VTK_USE_SYSTEM_JSONCPP, which is set by the vtk_module_third_party() call. When the call happens at the end, and VTK_USE_SYSTEM_JSONCPP is set to true, the rest of the file doesn't take this into account because the cache value hasn't been created yet. (You only see this on the first run of CMake; if you run it again the problem solves itself because the value is already in the cache.) This moves the call back to the beginning so that the rest of the file uses the correct value.