I have a slightly different way of doing this than the accepted answer. This way you can avoid using GROUP_CONCAT which has a limit of 1024 characters and will not work if you have a lot of fields.
SET @sql = '';
SELECT
@sql := CONCAT(@sql,if(@sql='','',', '),temp.output)
FROM
(
SELECT
DISTINCT
CONCAT(
'MAX(IF(pa.fieldname = ''',
fieldname,
''', pa.fieldvalue, NULL)) AS ',
fieldname
) as output
FROM
product_additional
) as temp;
SET @sql = CONCAT('SELECT p.id
, p.name
, p.description, ', @sql, '
FROM product p
LEFT JOIN product_additional AS pa
ON p.id = pa.id
GROUP BY p.id');
PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;