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/utils.h

@@ -11,6 +11,4 @@ char* itoa(int value, char* str, int base);
 // Convert unsigned integer to string (base is typically 10, 16, etc.)
 char* utoa(unsigned int value, char* str, int base);
 
-void *memset(void *dest, int value, size_t len);
-
 #endif // UTILS_H