Fix signed types warning in pasteboard handler (#2786)

imgui.cpp

@@ -9689,7 +9689,7 @@ static const char* GetClipboardTextFn_DefaultImpl(void*)
 
     ItemCount item_count = 0;
     PasteboardGetItemCount(main_clipboard, &item_count);
-    for (int i = 0; i < item_count; i++)
+    for (ItemCount i = 0; i < item_count; i++)
     {
         PasteboardItemID item_id = 0;
         PasteboardGetItemIdentifier(main_clipboard, i + 1, &item_id);