Fix build error with DEB_BUILD_HARDENING_FORMAT

Debian enables some hardening flags that causes a build error - More details here:

http://wiki.debian.org/Hardening

and specifically

http://wiki.debian.org/Hardening#DEB_BUILD_HARDENING_FORMAT_.28gcc.2Fg.2B-.2B-_-Wformat_-Wformat-security_-Werror.3Dformat-security.29