Fix a compiling error

The function fstat isn't declared without this include
on debian strect/sid and Fedora-23

Change-Id: I660a32ff173dcba04674aed51ed855b4fa55ac67
Signed-off-by: José Bollo <jose.bollo@iot.bzh>

tests/test_util_file.c

@@ -5,6 +5,7 @@
 #include <string.h>
 #include <fcntl.h>
 #include <unistd.h>
+#include <sys/stat.h>
 
 #include "json.h"
 #include "json_util.h"