Fix adding fields to mssql layers always resulted in error (fix #13340)

(cherry-picked from d827076)

Author: nyalldawson

src/providers/mssql/qgsmssqlprovider.cpp

@@ -998,6 +998,7 @@ bool QgsMssqlProvider::addAttributes( const QList<QgsField> &attributes )
     return false;
   }
 
+  loadFields();
   return true;
 }