netfilter: conntrack: use correct format characters

When compiling with -Wformat, clang emits the following warnings:

net/netfilter/nf_conntrack_helper.c:168:18: error: format string is not a string literal (potentially insecure) [-Werror,-Wformat-security]
                request_module(mod_name);
                               ^~~~~~~~

Use a string literal for the format string.

Link: https://github.com/ClangBuiltLinux/linux/issues/378
Signed-off-by: Bill Wendling <isanbard@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>