HAL_SITL: fixed build error

libraries/AP_HAL_SITL/HAL_SITL_Class.cpp

@@ -80,13 +80,17 @@ static char *new_argv[100];
 /*
   save watchdog data
  */
-static void watchdog_save(const uint32_t *data, uint32_t nwords)
+static bool watchdog_save(const uint32_t *data, uint32_t nwords)
 {
     int fd = ::open("persistent.dat", O_WRONLY|O_CREAT|O_TRUNC, 0644);
+    bool ret = false;
     if (fd != -1) {
-        ::write(fd, data, nwords*4);
+        if (::write(fd, data, nwords*4) == nwords*4) {
+            ret = true;
+        }
         ::close(fd);
     }
+    return ret;
 }
 
 /*