Include kstat.h from kmem.h

It turns out Solaris incidentally includes kstat.h from kmem.h.  As
a side effect of this certain higher level .c files which should
explicitly include kstat.h don't because they happen to get it
via kmem.h.  To make like easier for everyone I do the same.

include/sys/kmem.h

@@ -37,6 +37,7 @@
 #include <sys/types.h>
 #include <sys/debug.h>
 #include <sys/vmsystm.h>
+#include <sys/kstat.h>
 
 /*
  * Memory allocation interfaces