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>
(cherry picked from commit 37b9630d0a93ff2ffc89a85a095e41db72de1c5b)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 5d74de59f9428b2e1a3a8b58505bf9e7d187ea76)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>