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 {
      ^

llvm-svn: 335042
diff --git a/llvm/tools/llvm-exegesis/lib/Target.h b/llvm/tools/llvm-exegesis/lib/Target.h
index 3db49527..25b6671 100644
--- a/llvm/tools/llvm-exegesis/lib/Target.h
+++ b/llvm/tools/llvm-exegesis/lib/Target.h
@@ -29,7 +29,7 @@
   // Registers a target. Not thread safe.
   static void registerTarget(ExegesisTarget *T);
 
-  ~ExegesisTarget();
+  virtual ~ExegesisTarget();
 
 private:
   virtual bool matchesArch(llvm::Triple::ArchType Arch) const = 0;