libmemstat: unbreak build

r355706 added an instance of offsetof() to the UMA private kernel header
file uma_int.h. Userspace memstat_uma.c includes that header, and
chokes on offsetof() because apparently the definition in sys/types.h is
ifdef _KERNEL. Now, include sys/stddef.h which has an identical
definition.

Pointyhat to: rlibby
Sponsored by: Dell EMC Isilon