Replace Main->lock with an anoynous structure pointer

Replace Main->lock with an anoynous structure pointer

This way it's not needed to include BLI_threads.h from the
BKE_main.h which helps avoiding adding PThreads includes to
each library which uses Main on Windows.

From the API point of view it's now MainLock* and to lock or
unlock the main you're to use BKE_main_(un)lock().

This solves compilation error on Windows with SCons.