input_common/sdl: Remove unused header includes and forward declarations

Gets rid of a few unnecessary inclusion dependencies. It also uncovered
a few indirect inclusion dependencies being relied upon.

src/input_common/sdl/sdl.h

@@ -6,15 +6,8 @@
 
 #include <memory>
 #include <vector>
-#include "core/frontend/input.h"
 #include "input_common/main.h"
 
-union SDL_Event;
-
-namespace Common {
-class ParamPackage;
-} // namespace Common
-
 namespace InputCommon::Polling {
 class DevicePoller;
 enum class DeviceType;