scanner: Ignore C11 _Noreturn keyword

This shows up in the headers in FreeBSD's stdlib.  Now I didn't add
anything to regress.h because I think that'd introduce an effective
hard dependency on a C11-capable compiler, unless we used #ifdef
trickery.  But the problem with #ifdef is that the -expected.gir isn't
conditional.

https://bugzilla.gnome.org/show_bug.cgi?id=724851