Use (void) for function prototypes with no parameters

This is required when compiling these headers as C code. These were
omitted becuase before the addition of examples these headers were
only compiled locally as C++ files where adding the void keyword is
not needed.

include/SFML/Graphics/RenderStates.h

@@ -50,6 +50,6 @@ typedef struct
 /// \brief Define the default values for a RenderState
 ///
 ////////////////////////////////////////////////////////////
-CSFML_GRAPHICS_API sfRenderStates sfRenderStates_default();
+CSFML_GRAPHICS_API sfRenderStates sfRenderStates_default(void);
 
 #endif // SFML_RENDERSTATES_H