require feed debugging for frontend feed update button

tt-rss.php

@@ -226,9 +226,13 @@
 			<option value="score"><?php echo __('Score') ?></option>
 		</select>
 
+		<?php if (defined('_ENABLE_FEED_DEBUGGING')) { ?>
+
 		<button name="update" onclick="return viewCurrentFeed('ForceUpdate')">
 			<?php echo __('Update') ?></button>
 
+		<?php } ?>
+
 		<button onclick="return catchupCurrentFeed()">
 			<?php echo __('Mark as read') ?></button>