Basically you need to create also a Windows DLL which exports the same functions. These functions can be empty stubs if you don't want to run the code properly under Windows, otherwise you have to think about how to implement the equivalent functionality on the Windows API.
Then you can enter in your Call Library Node as library name "<your library name>.*" and name the Windows shared library "<your library name>.dll" while the Linux version is called "your library name>.so". LabVIEW will replace the * after the point in the library name with whatever is the prefered shared library extension for the platform it is running on and therefore load the right shared library.
Alternatively you could wrap all Call Library Nodes in a Conditional Compile structure that contains the Call Library Node in the Linux RT case and whatever you want to run on other platforms in the default case or a specific platform case.