Include compat/sys/kernel.h rather than "kernel.h".

Using "kernel.h" was pulling in the private kernel.h header instead,
which was causing a build failure on my branch since arch_cpu.h is
C++-only there.

src/libs/compat/freebsd_network/clock.c

@@ -5,7 +5,8 @@
 
 
 #include "device.h"
-#include "kernel.h"
+
+#include <compat/sys/kernel.h>
 
 
 int ticks;