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

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.

(cherry picked from commit 431a12cafeeec7300b7cea7e19b892d4e8c4900d)
Signed-off-by: Willy Tarreau <w@1wt.eu>