configure.ac: list radeon in --with-vulkan-drivers help string

Analogous to what we do for the dri and gallium drivers.

Cc: 17.0 13.0 <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.velikov@colllabora.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
(cherry picked from commit cb6be5c8c05fa1af20ebd4f014d686244826f987)
1 file changed