[ots] Make OTSContext destructor vertual
[ots] Make OTSContext destructor vertual

Since the class has virtual functions, its destructor ought to be
virtual.

Issue #41.

anthrotype/ots,davelab6/ots,irori/ots,khaledhosny/ots,davelab6/ots,irori/ots,anthrotype/ots