Replace override declarations with "virtual" so that we build on Ubuntu 12.04 .

Fixes #1336.

--
MOS_MIGRATED_REVID=124231262
diff --git a/src/main/cpp/blaze_util_mingw.cc b/src/main/cpp/blaze_util_mingw.cc
index ce0b684..fbcbb4e 100644
--- a/src/main/cpp/blaze_util_mingw.cc
+++ b/src/main/cpp/blaze_util_mingw.cc
@@ -295,7 +295,7 @@
  public:
   DummyBlazeServerStartup() {}
   virtual ~DummyBlazeServerStartup() {}
-  bool IsStillAlive() override { return true; }
+  virtual bool IsStillAlive() { return true; }
 };
 
 void ExecuteDaemon(const string& exe, const std::vector<string>& args_vector,