Kernel: clean up include statements a bit

Coverity was flagging a recursive include between kernel.h and
cpulocals.h. As cpulocals.h also included proc.h, we can move that
include statement into kernel.h, and clean up the source files'
include statements accordingly.

kernel/debug.c

@@ -4,8 +4,6 @@
  */
 
 #include "kernel.h"
-#include "proc.h"
-#include "debug.h"
 
 #include <minix/callnr.h>
 #include <minix/sysutil.h>