[Bug c++/69523] -Wliteral-suffix should not warn within namespace std

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69523

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
How about declaring those headers as system headers by using -isystem instead
of using -I :)?