Fix buildbot failures after r359159.

std::mutex was not actually is_nothrow_default_constructible in C++98/C++03,
because the variable declaration

  std::mutex M;

... could throw an exception from the mutex destructor. Fix it by marking the
destructor as non-throwing. This has no effect in C++11 onwards, because
destructors are non-throwing by default in those language modes.

git-svn-id: 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed