gfileutils: Make -Werror=format-nonliteral happy

I tried to please it by using G_GNUC_FORMAT, but that didn't work, so
let's just fall back to pushing an ignore.

https://bugzilla.gnome.org/show_bug.cgi?id=702516
More -Werror=format-nonliteral fixes

This fixes the build with CLang.

https://bugzilla.gnome.org/show_bug.cgi?id=702516