buildroot

lxc: fix offsetof() related musl build issue

This commit adds a patch that fixes a build issue with musl due to the
fact that offsetof() is used without including <stddef.h>.

Fixes:

  http://autobuild.buildroot.net/results/3c6e34cbcd43aba4dd2b18687bd89fc369c9dab7/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>