sys/font.h: avoid warnings about too-long initializer strings
Mark font_header::fh_magic as __non_string, to avoid a warning from
clang 21 similar to:
/usr/src/usr.bin/vtfontcvt/vtfontcvt.c:763:15: error: initializer-string for character array is too long, array size is 8 but initializer has size 9 (including the null terminating character); did you mean to use the 'nonstring' attribute? [-Werror,-Wunterminated-string-initialization]
763 | .fh_magic = FONT_HEADER_MAGIC,
| ^~~~~~~~~~~~~~~~~
/usr/obj/usr/src/amd64.amd64/tmp/usr/include/sys/font.h:109:27: note: expanded from macro 'FONT_HEADER_MAGIC'
109 | #define FONT_HEADER_MAGIC "VFNT0002"
| ^~~~~~~~~~MFC after: 3 days