use format string in ap_log_error

Debian/Ubuntu's gcc uses -Werror=format-security as default option, which
makes this warning a fatal error.
<http://wiki.debian.org/Hardening#DEB_BUILD_HARDENING_FORMAT_.28gcc.2Fg.2B-.2B-_-Wformat_-Wformat-security_-Werror.3Dformat-security.29>