Add a stubbed GCAdapter namespace.

This cleans up a bunch of #ifdef checks in places.

Source/Core/InputCommon/GCAdapter_Null.cpp

@@ -0,0 +1,23 @@
+// Copyright 2014 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#include "InputCommon/GCAdapter.h"
+namespace GCAdapter
+{
+void Init() {}
+void Reset() {}
+void ResetRumble() {}
+void Setup() {}
+void Shutdown() {}
+void SetAdapterCallback(std::function<void(void)> func) {}
+void StartScanThread() {}
+void StopScanThread() {}
+void Input(int chan, GCPadStatus* pad) {}
+void Output(int chan, u8 rumble_command) {}
+bool IsDetected() { return false; }
+bool IsDriverDetected() { return false; }
+bool DeviceConnected(int chan) { return false; }
+bool UseAdapter() { return false; }
+
+} // end of namespace GCAdapter