stable backport of "wireless: Use linux/stddef.h instead of stddef.h"

[Hauke Mehrtens <hauke@xxxxxxxxxx>]

Hi,

Please backport "wireless: Use linux/stddef.h instead of stddef.h" to 
kernel 4.14, 4.19 and 5.4.
This is upstream commit id 1b9ae0c92925ac40489be526d67d0010d0724ce0
https://git.kernel.org/linus/1b9ae0c92925ac40489be526d67d0010d0724ce0

commit 1b9ae0c92925ac40489be526d67d0010d0724ce0
Author: Hauke Mehrtens <hauke@xxxxxxxxxx>
Date:   Thu May 21 22:14:22 2020 +0200

    wireless: Use linux/stddef.h instead of stddef.h

This patch fixes a build problem in broken build environments which was 
introduced with 6989310f5d43 ("wireless: Use offsetof instead of custom 
macro.") which was backported to the listed kernel versions.

When the include path is fully correct you should not hit this problem, 
but I got it because of some bug in by build system and also someone 
else reported a similar problem to me and requested this backport.

Hauke

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