#399642 follow-up by JacobSingh: include /includes/install.inc in module_enable() which is required when enabling modules for the first time.

includes/module.inc

@@ -338,6 +338,8 @@ function module_enable($module_list, $enable_dependencies = TRUE, $disable_modul
 
   $invoke_modules = array();
 
+  // Required for _drupal_install_module().
+  include_once DRUPAL_ROOT . '/includes/install.inc';
   // Try to install the enabled modules and collect which were installed.
   // $module_list is not changed and already installed modules are ignored.
   $modules_installed = array_filter($module_list, '_drupal_install_module');