The full variant should conflict with the default variant. This prevents that
libgd and libgd-full could be installed side by side, and also, the full
variant should provide the libgd. Otherwise, if you install libgd-full,
you can not install vnstat.

Signed-off-by: Josef Schlehofer <pepe.schlehofer@gmail.com>
(cherry picked from commit 42b36b7180)