Grant MANAGE_USB permission, so we can get USB serial numbers

This is needed to reliably identify USB mass storage devices.
If someone has several identical thumb drives,
this prevents a backup from being performed when non-backup drives get attached.
diff --git a/permissions_com.stevesoltys.backup.xml b/permissions_com.stevesoltys.backup.xml
index a5ee731..34ed8dc 100644
--- a/permissions_com.stevesoltys.backup.xml
+++ b/permissions_com.stevesoltys.backup.xml
@@ -2,5 +2,6 @@
 <permissions>
     <privapp-permissions package="com.stevesoltys.backup">
         <permission name="android.permission.BACKUP"/>
+        <permission name="android.permission.MANAGE_USB"/>
     </privapp-permissions>
 </permissions>
\ No newline at end of file