xf86atomic: #undef internal define

Thanks to the #error just above, any file including this header can only
see one state for this macro: defined, with the value `1`.
Let's just #undef it once we're done using it in here so that other
files don't misconstrue any meaning to it.

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>