ControllerInterface: Add InputBackend interface and SDL implementation.

Source/Core/InputCommon/ControllerInterface/InputBackend.cpp

@@ -0,0 +1,24 @@
+// Copyright 2022 Dolphin Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#include "InputCommon/ControllerInterface/InputBackend.h"
+
+namespace ciface
+{
+InputBackend::InputBackend(ControllerInterface* controller_interface)
+    : m_controller_interface(*controller_interface)
+{
+}
+
+InputBackend::~InputBackend() = default;
+
+void InputBackend::UpdateInput()
+{
+}
+
+ControllerInterface& InputBackend::GetControllerInterface()
+{
+  return m_controller_interface;
+}
+
+}  // namespace ciface