exa: Fix a warning when enabling DEBUG_TRACE_FALL

The format string wants a picture and a character, but the argument list
contains only a character, causing GCC to complain. Add the missing
argument.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Keith Packard <keithp@keithp.com>