LinuxKPI: Add linux/stddef.h and add struct_group(...)

Merge the original addition of stddef.h.

(cherry picked from commit 7c7419f60cafa4f666f5a22b5e5f090081516cd2)

Add a struct_group() macro needed by a wireless driver.

Sponsored by: The FreeBSD Foundation
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D40138

(cherry picked from commit 7c0c69d1a0f3735e315a48436bffbd482f956270)