bcachefs-tools: Guard the __struct_group() #define in kernel.h
Our include/linux/byteorder.h uses the system <asm/byteorder.h>
because we don't provide our own in include/asm. This then pulls in
system <linux/stddef.h> which defines __struct_group. That definition
collides with the unconditional #define __struct_group in our local
include/linux/kernel.h, provoking preprocessor warnings.
Signed-off-by: Chris Webb <chris@arachsys.com> Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>