Fix deprecated notice about non-static function

set_request_last_from_entries() is being called statically when the function isn't declared static, resulting in this error:

> Deprecated: Non-static method WPCOM_Liveblog_AMP::set_request_last_from_entries() should not be called statically in /liveblog/classes/class-wpcom-liveblog-amp.php on line 259

This change fixes that by declaring the function as static.

classes/class-wpcom-liveblog-amp.php

@@ -270,7 +270,7 @@ public static function append_liveblog_to_content( $content ) {
 	 * @param  array  $entries liveblog entries.
 	 * @param  object $request Request Object.
 	 */
-	public function set_request_last_from_entries( $entries, $request ) {
+	public static function set_request_last_from_entries( $entries, $request ) {
 		if ( false === $request->last ) {
 			$request->last = $entries['entries'][0]->id . '-' . $entries['entries'][0]->timestamp;
 		}