On my Arch Linux system with doxygen 1.8.15-1, using line-wrapping backslashes
in \defgroup definitions ends up putting the backslash into the resulting .dot
file:

 digraph "VdpVideoMixer; Video Post-processing \"

graphviz doesn't like that:

 Error: /home/aaron/git/libvdpau/build/doc/html/group___vdp_video_mixer.dot: syntax error in line 1 near 'Helvetica'
 error: Problems running dot: exit code=1, command='dot', arguments='"/home/aaron/git/libvdpau/build/doc/html/group___vdp_video_mixer.dot" -Tpng -o "/home/aaron/git/libvdpau/build/doc/html/group___vdp_video_mixer.png"'

Fix this by just removing the line-wrapping even though these lines exceed 80
columns now.

Signed-off-by: Aaron Plattner <aplattner@nvidia.com>