Add instability attribute on private const_strlen function

Add instability attribute on private const_strlen function · rust-lang/rust@7f1518b

Original file line number	Diff line number	Diff line change
`@@ -740,6 +740,7 @@ impl AsRef<CStr> for CStr {`
`740`	`740`	`/// The pointer must point to a valid buffer that contains a NUL terminator. The NUL must be`
`741`	`741`	/// located within `isize::MAX` from `ptr`.
`742`	`742`	`#[inline]`
	`743`	`+#[unstable(feature = "cstr_internals", issue = "none")]`
`743`	`744`	`#[rustc_const_stable(feature = "const_cstr_from_ptr", since = "CURRENT_RUSTC_VERSION")]`
`744`	`745`	`#[rustc_allow_const_fn_unstable(const_eval_select)]`
`745`	`746`	`const unsafe fn const_strlen(ptr: *const c_char) -> usize {`