Add empty ontology-sections.txt.
Add empty ontology-sections.txt.

Looks like some gtk-doc versions really like this file, so build breaks
if it isn't there.

hoheinzollern/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,outofbits/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker