Change constructor visibility

Makes no sense to use a protected constructor here.. In fact, it restricts is from unit-testing this class

lib/SimpleSAML/Store/SQL.php

@@ -48,7 +48,7 @@ class SQL extends Store
     /**
      * Initialize the SQL data store.
      */
-    protected function __construct()
+    public function __construct()
     {
         $config = Configuration::getInstance();