gstreamer1.0-libav: disable API documentation

gstreamer1.0-libav: disable API documentation

With the latest gtk-doc release the gstreamer1.0-libav documentation fails to
build:

| gst-libav-plugins-docs.sgml:38: element include: XInclude error:
| could not load xml/tree_index.sgml, and no fallback was found

For the short term we can just disable the documentation.  The next upstream
release of gstreamer1.0-libav doesn't use gtk-doc anyway.

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>