libstd: io::fs::File::stat() need not to take &mut self.

The same goes for sys::fs::FileDesc::fstat() on Windows.
Signed-off-by: NNODA, Kai <nodakai@gmail.com>

src/libstd/io/fs.rs

@@ -237,7 +237,7 @@ pub fn eof(&self) -> bool {
     }
 
     /// Queries information about the underlying file.
-    pub fn stat(&mut self) -> IoResult<FileStat> {
+    pub fn stat(&self) -> IoResult<FileStat> {
         self.fd.fstat()
             .update_err("couldn't fstat file", |e|
                 format!("{}; path={}", e, self.path.display()))

src/libstd/sys/windows/fs.rs

@@ -131,7 +131,7 @@ pub fn truncate(&mut self, offset: i64) -> IoResult<()> {
         return ret;
     }
 
-    pub fn fstat(&mut self) -> IoResult<io::FileStat> {
+    pub fn fstat(&self) -> IoResult<io::FileStat> {
         let mut stat: libc::stat = unsafe { mem::zeroed() };
         match unsafe { libc::fstat(self.fd(), &mut stat) } {
             0 => Ok(mkstat(&stat)),