BUILD: http-htx: fix build warning regarding long type in printf

Commit a66adf41e ("MINOR: http-htx: Add understandable errors for the
errorfiles parsing") added a warning when loading malformed error files,
but this warning may trigger another build warning due to the %lu format
used. Let's simply cast it for output since it's just used for end user
output.

This must be backported to 2.0 like the commit above.
1 file changed