[cscore] Add USB camera change event (#3123)

cscore/src/main/native/osx/UsbCameraListener.cpp

@@ -0,0 +1,17 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#include "UsbCameraListener.h"
+
+using namespace cs;
+
+class UsbCameraListener::Impl {};
+
+UsbCameraListener::UsbCameraListener(wpi::Logger& logger, Notifier& notifier) {}
+
+UsbCameraListener::~UsbCameraListener() = default;
+
+void UsbCameraListener::Start() {}
+
+void UsbCameraListener::Stop() {}