Subject Kernel 5.12.0 build issue
From    Dilyn Corner 
Date    Fri, 7 May 2021 22:23:00 -0500

The launch of the 5.12.0 kernel resulted in a potential problem!  It's unclear
how many people this impacts; there have only been a few reports in IRC,
including me. There is a missing #include statement in one file.

A simple:
    sed -i '/<stdlib.h>/a #include <linux/stddef.h>' \

Should resolve the issue!

This persists through at least 5.12.2, and there are no obvious changes in
Linus' tree to indicate it's been reported or fixed.