cocoa: Fix -Wimplicit-retain-self warning in SDL_cocoawindow

SDL3/src/video/cocoa/SDL_cocoawindow.m:1149:38: error: block implicitly retains 'self'; explicitly mention 'self' to indicate this is intended behavior [-Werror,-Wimplicit-retain-self]
 1149 |         SDL_OnWindowLiveResizeUpdate(_data.window);

src/video/cocoa/SDL_cocoawindow.m

@@ -1146,7 +1146,7 @@ static NSCursor *Cocoa_GetDesiredCursor(void)
                                                       repeats:TRUE
                                                         block:^(NSTimer *unusedTimer)
     {
-        SDL_OnWindowLiveResizeUpdate(_data.window);
+        SDL_OnWindowLiveResizeUpdate(self->_data.window);
     }];
 
     [[NSRunLoop currentRunLoop] addTimer:liveResizeTimer forMode:NSRunLoopCommonModes];