make SDL_SetTextInputRect take a pointer to const

The documentation doesn't state that the argument is ever modified,
and no implementation does so currently.
This is a non-breaking change to guarantee as much to callers.

src/core/linux/SDL_fcitx.c

@@ -424,7 +424,7 @@ SDL_Fcitx_ProcessKeyEvent(Uint32 keysym, Uint32 keycode, Uint8 state)
 }
 
 void
-SDL_Fcitx_UpdateTextRect(SDL_Rect *rect)
+SDL_Fcitx_UpdateTextRect(const SDL_Rect *rect)
 {
     SDL_Window *focused_win = NULL;
     SDL_SysWMinfo info;