Thread: fix typo __Lockable.

git-svn-id: http://svn.boost.org/svn/boost/trunk@83551 b8fc166d-592f-0410-95f2-cb63ce0dd405
Thread: limit dependency on boost/thread/thread.hpp.

git-svn-id: http://svn.boost.org/svn/boost/trunk@83552 b8fc166d-592f-0410-95f2-cb63ce0dd405