Add stability attribute.

Author: bjoernager

library/core/src/char/methods.rs

@@ -673,6 +673,7 @@ impl char {
     /// 'ß'.encode_utf8(&mut b);
     /// ```
     #[stable(feature = "unicode_encode_char", since = "1.15.0")]
+    #[rustc_const_stable(feature = "const_char_encode_utf8", issue = "130512")]
     #[inline]
     pub const fn encode_utf8(self, dst: &mut [u8]) -> &mut str {
         // SAFETY: `char` is not a surrogate, so this is valid UTF-8.