Answer
In principle, to recite Qunoot in the Witr Salaat is Waajib. If one forgetfully omits a Waajib act, he must compensate for it by performing Sajdah-e-Sahw. If he did not make Sajdah-e-Sahw, he must repeat the Witr Salaat.
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai