doc: Automatically build the list of *-private.h files
doc: Automatically build the list of *-private.h files

Of course, each time one has to maintain a list of files by hand, it
tends to get out of synch. No exception here. Try to be slightly more
cunning and compute the list automatically.

(cherry picked from commit 10de693b69851497fa7b3238622b6502406c70dd)

Distrotech/cogl,Distrotech/cogl,Distrotech/cogl,Distrotech/cogl