Fix lint

Signed-off-by: Álvaro Brey <alvaro.brey@nextcloud.com>

app/src/main/res/layout/file_thumbnail.xml

@@ -26,6 +26,7 @@
 <FrameLayout xmlns:app="http://schemas.android.com/apk/res-auto"
     xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:tools="http://schemas.android.com/tools"
+    tools:ignore="MergeRootFrame"
     android:layout_width="wrap_content"
     android:layout_height="wrap_content"
     tools:showIn="@layout/list_item">