Why aren't client.ino and interrupt.ino one single .ino file? Why not make one single sketch that incorporates all the necessary code?
Anyway, your error is being thrown because you call the function interruptSetup() in client.ino, but you provide no function definition in client.ino. You do, however, define the function in the interrupt.ino file. You can't just divide up code into different sketches like that.