u_int is defined in sys/types.h
makes header usable alone.

--- sys/sys/sysctl.h.old	2012-05-25 11:08:53.000000000 -0400
+++ sys/sys/sysctl.h	2012-05-25 11:09:16.000000000 -0400
@@ -788,6 +788,7 @@
 		    struct sysctl_req *);
 #else	/* !_KERNEL */
 #include <sys/cdefs.h>
+#include <sys/types.h> /* for u_int */
 
 __BEGIN_DECLS
 int	sysctl(const int *, u_int, void *, size_t *, const void *, size_t);