Add dtor to InstX8632Lockable.

Speculative fix for Mac GCC build.

BUG=none
R=dschuff@chromium.org

Review URL: https://codereview.chromium.org/432523002
diff --git a/src/IceInstX8632.h b/src/IceInstX8632.h
index 7c12f11..fb57b75 100644
--- a/src/IceInstX8632.h
+++ b/src/IceInstX8632.h
@@ -621,6 +621,7 @@
     // with optimizations.
     HasSideEffects = Locked;
   }
+  virtual ~InstX8632Lockable() {}
 
 private:
   InstX8632Lockable(const InstX8632Lockable &) LLVM_DELETED_FUNCTION;