info: make _gst_debug_init() private for now

info: make _gst_debug_init() private for now

This was a FIXME for 0.11. I guess a case could be made to keep it around
separately for apps or libraries that only want to use GStreamer's debugging
system, but it seems more likely they'd just copy the two source files into
their own tree if the case. Also, things like types wouldn't be initialised
without gst_init(). We can still make it public again if anyone needs it,
but then we should make it a proper function and not hide it behind
underscores.