Auto merge of #26897 - RalfJung:stdin-mut, r=alexcrichton

This fixes #26890.

To be honest, the local compile-test is still running. This just takes so long. But this looks trivial enough...

Author: bors

src/libstd/io/stdio.rs

@@ -255,7 +255,7 @@ impl Stdin {
     //    in which case it will wait for the Enter key to be pressed before
     ///   continuing
     #[stable(feature = "rust1", since = "1.0.0")]
-    pub fn read_line(&mut self, buf: &mut String) -> io::Result<usize> {
+    pub fn read_line(&self, buf: &mut String) -> io::Result<usize> {
         self.lock().read_line(buf)
     }
 }