include: Add UI_ANIMATION_KEYFRAME_STORYBOARD_START constant.

include/uianimation.idl

@@ -30,6 +30,7 @@ interface IUIAnimationTransitionLibrary;
 
 typedef DOUBLE UI_ANIMATION_SECONDS;
 typedef struct { int _; } *UI_ANIMATION_KEYFRAME;
+const UI_ANIMATION_KEYFRAME UI_ANIMATION_KEYFRAME_STORYBOARD_START = (UI_ANIMATION_KEYFRAME)(-1);
 
 const UI_ANIMATION_SECONDS UI_ANIMATION_SECONDS_EVENTUALLY = -1.0;