NFC: Add _Null_unspecified _swift_stdlib_getEnviron() declaration.

Makes the BSD and `__APPLE__` branches consistent and suppresses the following
warning:

```
warning: pointer is missing a nullability type specifier
```

Author: tshortli

stdlib/public/SwiftShims/swift/shims/LibcOverlayShims.h

@@ -59,7 +59,7 @@ static inline char * _Nullable * _Null_unspecified _swift_stdlib_getEnviron() {
   return environ;
 }
 #elif defined(__APPLE__)
-static inline char * _Nullable *_swift_stdlib_getEnviron() {
+static inline char * _Nullable * _Null_unspecified _swift_stdlib_getEnviron() {
   extern char * _Nullable **_NSGetEnviron(void);
   return *_NSGetEnviron();
 }