login_page needs to pull in install_helper_functions_api

To read schema.php, a function definition for db_null_date() is
required. This function is defined within
install_helper_functions_api.php.

Therefore login_page.php needs to include install_helper_functions_api
prior to reading the schema.

login_page.php

@@ -251,6 +251,8 @@
 		}
 
 		# Check for db upgrade for versions > 1.0.0 using new installer and schema
+		# Note: install_helper_functions_api.php required for db_null_date() function definition
+		require_api( 'install_helper_functions_api.php' );
 		require_once( 'admin' . DIRECTORY_SEPARATOR . 'schema.php' );
 		$t_upgrades_reqd = count( $upgrade ) - 1;