Added a missing call to 'Reset'.
HostThreadWindows::Join() did not call the Reset as is done by
the HostThreadPosix::Join(). As a result, future call to 
IsJoinable() can fail.

Committed as obvious.
 


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@220651 91177308-0d34-0410-b5e6-96231b3b80d8