Don't require external use of internal enum smb_thread_lock_type

Don't require external use of internal enum smb_thread_lock_type

- Internally, when locking or unlocking a mutex, we'll pass one of the
  values of enum smb_thread_lock_type. That enum is not available to users
  providing a thread implementation. Externally, we'll document the integer
  values which will be passed to their lock_mutex function, but not require
  them to access our internal header file.

Derrell