Fix virtual class with non-virtual dtor complaint

Fixes compiler complaint about:
src/Unwind/UnwindCursor.hpp:366:25: error: 'libunwind::AbstractUnwindCursor' has virtual functions but non-virtual destructor [-Werror,-Wnon-virtual-dtor]


git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@206942 91177308-0d34-0410-b5e6-96231b3b80d8