libmgba: Fix build when disabled

Source/Core/Core/HW/GBACore.cpp

@@ -1,6 +1,8 @@
 // Copyright 2021 Dolphin Emulator Project
 // SPDX-License-Identifier: GPL-2.0-or-later
 
+#ifdef HAS_LIBMGBA
+
 #include "Core/HW/GBACore.h"
 
 #define PYCPARSE  // Remove static functions from the header
@@ -753,3 +755,4 @@ std::string Core::GetSavePath(std::string_view rom_path, int device_number)
   return save_path;
 }
 }  // namespace HW::GBA
+#endif  // HAS_LIBMGBA

Source/Core/Core/HW/GBACore.h

@@ -3,6 +3,8 @@
 
 #pragma once
 
+#ifdef HAS_LIBMGBA
+
 #include <array>
 #include <condition_variable>
 #include <memory>
@@ -147,3 +149,4 @@ private:
   ::Core::System& m_system;
 };
 }  // namespace HW::GBA
+#endif  // HAS_LIBMGBA

Source/Core/Core/HW/SI/SI_DeviceGBAEmu.cpp

@@ -1,6 +1,8 @@
 // Copyright 2021 Dolphin Emulator Project
 // SPDX-License-Identifier: GPL-2.0-or-later
 
+#ifdef HAS_LIBMGBA
+
 #include "Core/HW/SI/SI_DeviceGBAEmu.h"
 
 #include <vector>
@@ -174,3 +176,4 @@ void CSIDevice_GBAEmu::OnEvent(u64 userdata, s64 cycles_late)
   m_system.GetSerialInterface().ScheduleEvent(m_device_number, num_cycles);
 }
 }  // namespace SerialInterface
+#endif  // HAS_LIBMGBA

Source/Core/Core/HW/SI/SI_DeviceGBAEmu.h

@@ -3,6 +3,8 @@
 
 #pragma once
 
+#ifdef HAS_LIBMGBA
+
 #include <memory>
 
 #include "Common/CommonTypes.h"
@@ -47,3 +49,4 @@ private:
   std::shared_ptr<GBAHostInterface> m_gbahost;
 };
 }  // namespace SerialInterface
+#endif  // HAS_LIBMGBA