Fix compiler warning (incompatible pointer type).
Fix compiler warning (incompatible pointer type).

tonypaulic/xfce4-genmon-plugin,dnschneid/xfce4-genmon-plugin,dnschneid/xfce4-genmon-plugin,xfce-mirror/xfce4-genmon-plugin,tonypaulic/xfce4-genmon-plugin,tonypaulic/xfce4-genmon-plugin,dnschneid/xfce4-genmon-plugin,xfce-mirror/xfce4-genmon-plugin,xfce-mirror/xfce4-genmon-plugin