add Chrono.h to patch around a gcc 4.6 issue

Test Plan: used it

Reviewed By: soren@fb.com

FB internal diff: D859244

folly/Chrono.h

+/*
+ * Copyright 2013 Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+// Wrapper around <chrono> that hides away some gcc 4.6 issues
+#ifndef FOLLY_CHRONO_H_
+#define FOLLY_CHRONO_H_
+
+#include <chrono>
+#include "folly/Portability.h"
+
+// gcc 4.6 uses an obsolete name for steady_clock, although the implementation
+// is the same
+#if __GNUC_PREREQ(4, 6) && !__GNUC_PREREQ(4, 7)
+namespace std { namespace chrono {
+typedef monotonic_clock steady_clock;
+}}  // namespaces
+#endif
+
+#endif /* FOLLY_CHRONO_H_ */
+