fix some compiler warnings

cpu/native/atomic_cpu.c

@@ -14,8 +14,8 @@
  */
 
 #include <atomic.h>
-#include <cpu.h>
-#include "debug.h"
+#include <irq.h>
+#include <debug.h>
 
 unsigned int atomic_set_return(unsigned int* val, unsigned int set)
 {

cpu/native/lpm_cpu.c

@@ -14,6 +14,7 @@
  */
 #include <stdio.h>
 #include <stdlib.h>
+#include <unistd.h>
 
 #include "lpm.h"
 #include "debug.h"

cpu/native/rtc/posix-rtc.c

@@ -14,6 +14,7 @@
  */
 
 #include <time.h>
+#include <err.h>
 
 #include "debug.h"
 

cpu/native/startup.c

@@ -15,8 +15,11 @@
 
 #include <stdio.h>
 #include <kernel_intern.h>
+#include <cpu.h>
 
 extern void board_init(void);
+extern void native_cpu_init(void);
+extern void native_interrupt_init(void);
 
 __attribute__ ((constructor)) static void startup(void)
 {