Mark another test as flaky

Reported on the NetBSD buildbot.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@353622 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/thread/thread.mutex/thread.mutex.requirements/thread.shared_mutex.requirements/thread.shared_mutex.class/try_lock.pass.cpp

@@ -9,6 +9,8 @@
 // UNSUPPORTED: libcpp-has-no-threads
 // UNSUPPORTED: c++98, c++03, c++11, c++14
 
+// FLAKY_TEST.
+
 // <shared_mutex>
 
 // class shared_mutex;