Don't ship headers of all the libraries

Submitted by Sebastian Dröge (slomo)

Description

We currently ship headers for every single library we build, including for example all the codec libraries. This seems unneeded and confusing and we should stop doing that :)

IMHO we should only ship those headers that are required for GStreamer and for those libraries where we need the application to have the headers to make use of the corresponding plugin (cairo and libnice, anything else?).