Add a missing "once" in .h

Summary: Closes https://github.com/facebook/rocksdb/pull/2670

Differential Revision: D5529018

Pulled By: siying

fbshipit-source-id: 10a378933d509035d2dbe502247dd85fcea09789

util/cast_util.h

@@ -3,6 +3,8 @@
 //  COPYING file in the root directory) and Apache 2.0 License
 //  (found in the LICENSE.Apache file in the root directory).
 
+#pragma once
+
 namespace rocksdb {
 // The helper function to assert the move from dynamic_cast<> to
 // static_cast<> is correct. This function is to deal with legacy code.