LibCore: Implement System::fchown

Author: https://github.com/sin-ack
Commit: 37658e6fa6
Pull-request: https://github.com/SerenityOS/serenity/pull/12319

Userland/Libraries/LibCore/System.h

@@ -114,6 +114,7 @@ ErrorOr<void> chdir(StringView path);
 ErrorOr<pid_t> fork();
 ErrorOr<int> mkstemp(Span<char> pattern);
 ErrorOr<void> fchmod(int fd, mode_t mode);
+ErrorOr<void> fchown(int fd, uid_t, gid_t);
 ErrorOr<void> rename(StringView old_path, StringView new_path);
 ErrorOr<void> unlink(StringView path);
 ErrorOr<void> utime(StringView path, Optional<struct utimbuf>);