Fixed a fatal error when compiled with -Werror=format-security.

Fixed a fatal error when compiled with -Werror=format-security.

This change is a workaround a fatal error that we get when compiling
sysstat with -Werror=format-securit.

Mail from Guillaume Rousse (guillomovitch@gmail.com) 30/07/2012:

Voici un patch que j'ai retrouvé dans le package mageia de sysstat, qui corrige une erreur d'utilisation de printf (fatale avec les options -Wformat -Werror=format-security).
[...]

(Following patch doesn't work properly).