Fix missing #includes in system/core.

Change-Id: I4736e8c4f7bb8bb03640d26d58243efaa67dc75c
diff --git a/toolbox/uptime.c b/toolbox/uptime.c
index 3fb4606..2dd8084 100644
--- a/toolbox/uptime.c
+++ b/toolbox/uptime.c
@@ -36,7 +36,7 @@
 #include <fcntl.h>
 #include <stdio.h>
 #include <time.h>
-
+#include <unistd.h>
 
 static void format_time(int time, char* buffer) {
     int seconds, minutes, hours, days;