Fix PDE/PTE definitions, header cleanup

- Fixes PDE/PTE definitions in kernel/paging.h
- removes memset declaration from kernel/utils.h, uses klibc string.h as
needed

kernel/threading.c

@@ -1,9 +1,8 @@
+#include <stdbool.h>
+#include <string.h>
 #include "malloc.h"
 #include "print.h"
 #include "threading.h"
-#include "types.h"
-#include "utils.h"
-#include <stdint.h>
 
 #define MAX_THREADS 16           // Maximum number of threads
 #define THREAD_STACK_SIZE 8192   // Stack size for each thread