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>

configure.ac

@@ -1738,7 +1738,7 @@ fi
 AC_ARG_WITH([vulkan-drivers],
     [AS_HELP_STRING([--with-vulkan-drivers@<:@=DIRS...@:>@],
         [comma delimited Vulkan drivers list, e.g.
-        "intel"
+        "intel,radeon"
         @<:@default=no@:>@])],
     [with_vulkan_drivers="$withval"],
     [with_vulkan_drivers="no"])