Adds declare statement back on non-problematic files

Author: AltamashShaikh

ScheduleReportSlack.php

@@ -7,6 +7,8 @@
  * @license https://www.gnu.org/licenses/gpl-3.0.html GPL v3 or later
  */
 
+declare(strict_types=1);
+
 namespace Piwik\Plugins\Slack;
 
 class ScheduleReportSlack

SlackApi.php

@@ -7,6 +7,8 @@
  * @license https://www.gnu.org/licenses/gpl-3.0.html GPL v3 or later
  */
 
+declare(strict_types=1);
+
 namespace Piwik\Plugins\Slack;
 
 use Piwik\Container\StaticContainer;