core/xfa: make const XFA's volatile
core/xfa: make const XFA's volatile

This prevents gcc from figuring out that an XFA that has been
initialized in the same file is technically empty when the compilation
unit is seen by itself. This happened with gcc 10.1.0 on msp430-elf.

authmillenon/RIOT,kYc0o/RIOT,jasonatran/RIOT,OlegHahm/RIOT,ant9000/RIOT,ant9000/RIOT,miri64/RIOT,kaspar030/RIOT,kYc0o/RIOT,OlegHahm/RIOT,ant9000/RIOT,jasonatran/RIOT,jasonatran/RIOT,OTAkeys/RIOT,kYc0o/RIOT,authmillenon/RIOT,kaspar030/RIOT,kaspar030/RIOT,OTAkeys/RIOT,miri64/RIOT,authmillenon/RIOT,kaspar030/RIOT,kYc0o/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,ant9000/RIOT,OlegHahm/RIOT,miri64/RIOT,ant9000/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,authmillenon/RIOT,authmillenon/RIOT,OlegHahm/RIOT,jasonatran/RIOT,kaspar030/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,miri64/RIOT,OlegHahm/RIOT,jasonatran/RIOT,miri64/RIOT