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

lib/libmemstat/memstat_uma.c

@@ -31,6 +31,7 @@
 #include <sys/param.h>
 #include <sys/counter.h>
 #include <sys/cpuset.h>
+#include <sys/stddef.h>
 #include <sys/sysctl.h>
 
 #include <vm/uma.h>