Use "struct SDL_GamePad*" for C compatibility (#83)

Author: dbalatoni13

include/dolphin/pad.h

@@ -157,7 +157,7 @@ const char* PADGetNativeAxisName(PADSignedNativeAxis axis);
 /**
  * Returns the SDL gamepad for the index into the controller map.
  */
-SDL_Gamepad* PADGetSDLGamepadForIndex(u32 index);
+struct SDL_Gamepad* PADGetSDLGamepadForIndex(u32 index);
 /* Returns the first native button which is currently pressed */
 s32 PADGetNativeButtonPressed(u32 port);
 /* Returns the first native axis which is currently pulled halfway or more */