waffle_dl: Fix missing format string parameter
waffle_dl: Fix missing format string parameter

Previously, this would print whatever uninitialized value happened to
be on the stack.

Signed-off-by: Simon McVittie <0e1aea601907c1a85c2dc96e5ea37e23c58d2e9b@collabora.com>
Reviewed-by: Emil Velikov <4c996f2f093ddd0585a556460b4cc3ba555546ef@gmail.com>

waffle-gl/waffle,waffle-gl/waffle,waffle-gl/waffle,waffle-gl/waffle