libc: Move the declaration of futimens from sys/time.h to sys/stat.h

to follow the spec here:
https://pubs.opengroup.org/onlinepubs/9699919799/functions/futimens.html

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>
Change-Id: Id8e418794bd43827c01af772ce704a8d3e7681c5

libs/libc/unistd/lib_futimens.c

@@ -24,13 +24,32 @@
 
 #include <nuttx/config.h>
 
-#include <sys/time.h>
+#include <sys/stat.h>
 #include <errno.h>
 
 /****************************************************************************
  * Public Functions
  ****************************************************************************/
 
+/****************************************************************************
+ * Name: futimens
+ *
+ * Description:
+ *   futimens() update the timestamps of a file with nanosecond precision.
+ *   This contrasts with the historical utime(2) and utimes(2), which permit
+ *   only second and microsecond precision, respectively, when setting file
+ *   timestamps.
+ *
+ * Input Parameters:
+ *   fd  - Specifies the fd to be modified
+ *   times - Specifies the time value to set
+ *
+ * Returned Value:
+ *   On success, futimens() return 0.
+ *   On error, -1 is returned and errno is set to indicate the error.
+ *
+ ****************************************************************************/
+
 int futimens(int fd, const struct timespec times[2])
 {
   set_errno(ENOTSUP);