[build] Don't dist gtk-doc.m4
[build] Don't dist gtk-doc.m4

As a rule of thumb, we should only add to EXTRA_DIST the m4 files we
have in the git repository.

GNOME/clutter-gst,skinkie/clutter-gst,lubosz/clutter-gst,ystreet/clutter-gst,lubosz/clutter-gst,GNOME/clutter-gst,ystreet/clutter-gst,GNOME/clutter-gst,skinkie/clutter-gst,skinkie/clutter-gst,GNOME/clutter-gst,lubosz/clutter-gst,ystreet/clutter-gst,ystreet/clutter-gst