Make rpmplugins.h private for now

Make rpmplugins.h private for now
- We'll probably want to make some changes to the plugin type system
  before considered "ready", the current plugin slots are limited
  to 32 and Meego folks apparently want to use a largish number
  of slots. So we might want something like separate plugin type
  (collection, security... etc) and then have 32 slots per each type.
  Making this private for now to avoid having to potentially break
  the API shortly after introduction.