doc: Keep SECTION: after the ifdef
doc: Keep SECTION: after the ifdef

Otherwise GTK doc will see it as often as we include that files
and warn about duplicated SECTION:

krichter722/gstreamer,shelsonjava/gstreamer,centricular/gstreamer,jpxiong/gstreamer,krichter722/gstreamer,jpxiong/gstreamer,shelsonjava/gstreamer,krichter722/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,centricular/gstreamer,shelsonjava/gstreamer,shelsonjava/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,jpxiong/gstreamer,shelsonjava/gstreamer,jpxiong/gstreamer,centricular/gstreamer,krichter722/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer