libphobos: Check in missing core.sync package module

This was meant to be part of r13-1062 in the merge with upstream
druntime 454471d8.

libphobos/libdruntime/core/sync/package.d

+/**
+ * Provides thread synchronization tools such as mutexes, semaphores and barriers.
+ *
+ * License: Distributed under the
+ *      $(LINK2 http://www.boost.org/LICENSE_1_0.txt, Boost Software License 1.0).
+ *    (See accompanying file LICENSE)
+ * Authors:   Sean Kelly, Rainer Schuetze
+ * Source:    $(DRUNTIMESRC core/sync/package.d)
+ */
+
+module core.sync;
+
+public import core.sync.barrier;
+public import core.sync.condition;
+public import core.sync.config;
+public import core.sync.event;
+public import core.sync.exception;
+public import core.sync.mutex;
+public import core.sync.rwmutex;
+public import core.sync.semaphore;