doxygen: Correct TAGFILE relative paths

doxygen: Correct TAGFILE relative paths

Per Doxygen documentation, to combine external documentation (stored in
a *.tag file) with a project the TAGFILES option should be set in the
configuration file.

  A tag file typically only contains a relative location of the
  documentation from the point where doxygen was run. So when
  you include a tag file in other project you have to specify
  where the external documentation is located in relation this
  project.

  You can do this in the configuration file by assigning the
  (relative) location to the tag files specified after the
  TAGFILES configuration option.

  If you use a relative path it should be relative with respect
  to the directory where the HTML output of your project is
  generated; so a relative path from the HTML output directory
  of a project to the HTML output of the other project that is
  linked to.

Signed-off-by: Rhys Kidd <rhyskidd@gmail.com>
Acked-by: Emil Velikov <emil.velikov@collabora.com>