lib/tst_kconfig: Include stddef.h #332

Job	Run time
job (debian:stable, gcc, i386)	10m 0s
job (debian:stable, ppc64el, powerpc64le-linux-gnu-gcc, 1, out, cross-compile)	5m 4s
job (debian:stable, arm64, aarch64-linux-gnu-gcc, 1, out, cross-compile)	5m 0s
job (debian:stable, s390x, s390x-linux-gnu-gcc, 1, out, cross-compile)	4m 49s
job (alpine:latest, gcc, asciidoctor)	10m 12s
job (debian:stable, gcc, out, minimal)	6m 58s
job (opensuse/archive:42.2, gcc)	7m 54s
job (quay.io/centos/centos:stream9, gcc, asciidoc-pdf, out)	9m 22s
job (debian:testing, gcc, asciidoctor-pdf)	10m 54s
job (debian:oldstable, clang)	11m 30s
job (fedora:latest, clang, 1, asciidoctor-pdf)	8m 27s
job (opensuse/leap:latest, gcc, asciidoc-pdf)	10m 36s
job (debian:oldstable, gcc, asciidoctor)	8m 41s
job (debian:testing, clang, asciidoctor-pdf)	12m 41s
job (ubuntu:jammy, gcc, asciidoctor, out)	4m 59s
job (ubuntu:bionic, gcc, asciidoc-pdf)	9m 58s
	2h 17m 5s

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