Add mach_error_string (and mach_error_t) (rust-lang#3913)

`mach_error_string` is defined in `/usr/include/mach/mach_error.h`
It is not referenced in the documentation Apple website.

```
char            *mach_error_string(
/*
 *  Returns a string appropriate to the error argument given
 */
    mach_error_t error_value
    );
```

`mach_error_t` is defined in `/usr/include/mach/error.h`
https://developer.apple.com/documentation/kernel/mach_error_t

```
typedef kern_return_t   mach_error_t;
```

(backport <rust-lang#3913>)
(cherry picked from commit d8ff07b)

Author: stepancheg

src/unix/bsd/apple/mod.rs

@@ -6240,6 +6240,8 @@ extern "C" {
 
     pub fn mach_error_string(error_value: crate::mach_error_t) -> *mut c_char;
 
+    pub fn mach_error_string(error_value: ::mach_error_t) -> *mut ::c_char;
+
     // Added in macOS 10.13
     // ISO/IEC 9899:2011 ("ISO C11") K.3.7.4.1
     pub fn memset_s(s: *mut c_void, smax: size_t, c: c_int, n: size_t) -> c_int;