Pull in an updated version of stddef.h from the linux kernel.
Pulled from upstream kernel at 891003abb0db6bfffd61b76ad0ed39bb7c3db8e1
This file was generated using the following command:
cd bionic/libc/kernel/
./tools/clean_header.py -u ../../../external/kernel-headers/original/linux/stddef.h
Change-Id: I6c29f3fa100c5368da41d0f0da39bc50fa668e9d