docs: exclude more header from doc-scan
docs: exclude more header from doc-scan

Exclude gst/printf/*.h and a few generated .h files from the api-scan. This
makes -unused.txt report file from gtkdoc useful again.

StreamUtils/gstreamer,centricular/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,centricular/gstreamer