doc: disable dot graph in doxygen

doc: disable dot graph in doxygen

Ubuntu ships with a patched version of doxygen that enables
HAVE_DOT (disabled by default). Enabling this option causes the warning:
"""
warning: Included by graph for 'rte_common.h' not generated,
too many nodes. Consider increasing DOT_GRAPH_MAX_NODES
"""
This reproduces with doxygen version 1.8.13 and
dot - graphviz version 2.40.1 on Ubuntu 18.04.

This will force doxygen not to assume that dot (part of Graphviz)
is installed, and will result in dot not being used for visualization.

If someone still needs to generate the graphs, the following can
be considered:
- Increase DOT_GRAPH_MAX_NODES to a large value.
- Set HAVE_DOT for more powerful graphs.
- Set DOT_IMAGE_FORMAT=svg to generate svg images.
- Set INTERACTIVE_SVG=YES to allow zooming and panning.

See:
- http://changelogs.ubuntu.com/changelogs/pool/main/d/doxygen/doxygen_1.8.13-10/changelog
- http://www.doxygen.nl/manual/config.html#cfg_have_dot
- https://github.com/doxygen/doxygen/issues/7345

Signed-off-by: Ali Alnubani <alialnu@mellanox.com>
Acked-by: John McNamara <john.mcnamara@intel.com>