llvm-exegesis: mark ~ExegesisTarget() as virtual. Fixes build.

/build/llvm/tools/llvm-exegesis/lib/X86/../Target.h:32:3: error: 'exegesis::ExegesisTarget' has virtual functions but non-virtual destructor [-Werror,-Wnon-virtual-dtor]
  ~ExegesisTarget();
  ^
/build/llvm/tools/llvm-exegesis/lib/X86/Target.cpp:15:7: error: 'exegesis::(anonymous namespace)::ExegesisX86Target' has virtual functions but non-virtual destructor [-Werror,-Wnon-virtual-dtor]
class ExegesisX86Target : public ExegesisTarget {
      ^

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@335042 91177308-0d34-0410-b5e6-96231b3b80d8