Fix missing namespace

Author: raphaelm

pretixprint/app/src/main/res/layout/fragment_usb_settings.xml

@@ -1,6 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:app="http://schemas.android.com/apk/res-auto"
+    xmlns:tools="http://schemas.android.com/tools"
     android:id="@+id/root"
     android:layout_width="match_parent"
     android:layout_height="match_parent"