Coexistence of multiple gastrointestinal stromal tumors and signet ring cell carcinoma of stomach in a patient with Neurofibromatosis Type-1: Case report Nörofibromatozis Tip-1'li hastada multipl gastrointestinal stromal tümör ve midede taşlı yüzük hücreli karsinom birlikteliǧi: Olgu sunumu