Disable automatic backups when a removable USB flash drive is used

as storage location.

The backup backoff time is not reliable for this as the system still
attempts to backup the magic @pm@ package without checking for the
backoff value.
diff --git a/permissions_com.stevesoltys.backup.xml b/permissions_com.stevesoltys.backup.xml
index 34ed8dc..f019538 100644
--- a/permissions_com.stevesoltys.backup.xml
+++ b/permissions_com.stevesoltys.backup.xml
@@ -3,5 +3,6 @@
     <privapp-permissions package="com.stevesoltys.backup">
         <permission name="android.permission.BACKUP"/>
         <permission name="android.permission.MANAGE_USB"/>
+        <permission name="android.permission.WRITE_SECURE_SETTINGS"/>
     </privapp-permissions>
 </permissions>
\ No newline at end of file