Add empty virtual destructor to class with virtual methods.

Clang 3.5 has a warning about deleting objects with virtual methods
through non-virtual destructors which was triggered. I'm not sure
whether this actually creates any undefined or otherwise incorrect
behavior, but it seems like a good thing to fix regardless.

Example compiler warning:
third_party/gperftools/src/tests/profile-handler_unittest.cc:282:5: error:
  delete called on '(anonymous namespace)::BusyThread' that has virtual
  functions but non-virtual destructor [-Wdelete-non-virtual-dtor]
    delete busy_worker_;
    ^

1 files changed, 1 insertions, 0 deletions

diff --git a/src/tests/profile-handler_unittest.cc b/src/tests/profile-handler_unittest.cc
index 72a4640..2984d0d 100644
--- a/src/tests/profile-handler_unittest.cc
+++ b/src/tests/profile-handler_unittest.cc
@@ -45,6 +45,7 @@ namespace {
 class Thread {
  public:
   Thread() : joinable_(false) { }
+  virtual ~Thread() { }
   void SetJoinable(bool value) { joinable_ = value; }
   void Start() {
     pthread_attr_t attr;