Validation require memcached extension for distributed pollers (#7815)

LibreNMS/Validations/Php.php

@@ -81,6 +81,11 @@ class Php implements ValidationGroup
     private function checkExtensions(Validator $validator)
     {
         $required_modules = array('mysqli','pcre','curl','session','snmp','mcrypt', 'xml', 'gd');
+
+        if (Config::get('distributed_poller')) {
+            $required_modules[] = 'memcached';
+        }
+
         foreach ($required_modules as $extension) {
             if (!extension_loaded($extension)) {
                 $validator->fail("Missing PHP extension: $extension", "Please install $extension");