Add -Wformat-security to --enable-warnings=fatal.
Add -Wformat-security to --enable-warnings=fatal.

Because we use it in glibmm too. No code changes were necessary.

GNOME/libxmlmm,GNOME/libxmlmm,GNOME/libxmlmm,dtrebbien/libxmlmm,dtrebbien/libxmlmm,GNOME/libxmlmm,dtrebbien/libxmlmm