Fix a gcc warning:

Fix a gcc warning:

'class llvm::DAGDeltaAlgorithm' has virtual functions and accessible non-virtual destructor

Not sure if this is the best solution, but this class has state and some of the
classes that inherit from it also do, so it looks appropriate.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@105675 91177308-0d34-0410-b5e6-96231b3b80d8