want sys/stddef.h for offsetof and container_of macros
The current uts/common/sys/sysmacros.h does attempt to protect offsetof() and container_of() macros from getting exposed to userland stddef.h by using construct:
#if (defined(_KERNEL) || defined(_FAKE_KERNEL)) && !defined(_KMEMUSER) .. #endif
However, the i86pc and i86xpv dboot is built with -D_BOOT -D_KMEMUSER -U_KERNEL and is therefore excluded from offsetof() and container_of() macros. To solve this issue, we need to update the conditional expression.