make struct _GThread private

Hide the definition of struct _GThread in gthreadprivate.h for now.

This is possibly an API break -- although the structure contents were
undocumented and it was not safe to access them in a meaningful way,
someone may have tried to do it anyway.  We'll leave it here for a while
to see if it causes any problems.

Avoid merging its contents with GRealThread for now, just incase we need
to expose it again.