InputCommon: Move SDL Device class into its own cpp/h files.

Source/Core/InputCommon/CMakeLists.txt

@@ -177,6 +177,8 @@ if(ENABLE_SDL)
   target_sources(inputcommon PRIVATE
     ControllerInterface/SDL/SDL.cpp
     ControllerInterface/SDL/SDL.h
+    ControllerInterface/SDL/SDLGamepad.cpp
+    ControllerInterface/SDL/SDLGamepad.h
   )
   target_link_libraries(inputcommon PRIVATE SDL2::SDL2)
   target_compile_definitions(inputcommon PUBLIC HAVE_SDL2=1)