Include <sys/mutex.h> and its prerequisite <sys/lock.h> instead of

depending on namespace pollution in <sys/vnode.h> for the definition
of GIANT_REQUIRED.

Sorted includes.

sys/compat/linux/linux_util.c

@@ -33,10 +33,12 @@
 __FBSDID("$FreeBSD$");
 
 #include <sys/param.h>
-#include <sys/systm.h>
+#include <sys/lock.h>
+#include <sys/malloc.h>
+#include <sys/mutex.h>
 #include <sys/namei.h>
 #include <sys/proc.h>
-#include <sys/malloc.h>
+#include <sys/systm.h>
 #include <sys/vnode.h>
 
 #include <machine/stdarg.h>