Randomized, open-label, controlled trial to assess the clinical efficacy and safety of meloxicam suspension 0.25 mg/kg/day once a day, versus diclofenac 1 mg/kg/day twice a day or nimesulide 4 mg/kg/day twice a day, for five days in the treatment of patients with acute, non-bacterial pharyngitis, pharyngotonsillitis or laryngitis