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/video/uikit/SDL_uikitviewcontroller.m

@@ -547,7 +547,7 @@ UIKit_IsScreenKeyboardShown(_THIS, SDL_Window *window)
 }
 
 void
-UIKit_SetTextInputRect(_THIS, SDL_Rect *rect)
+UIKit_SetTextInputRect(_THIS, const SDL_Rect *rect)
 {
     if (!rect) {
         SDL_InvalidParamError("rect");