hurd: Add XATTR_CREATE, XATTR_REPLACE

(backport <rust-lang#3739>)
(cherry picked from commit 9d33ec2)

Author: sthibaul

src/unix/hurd/mod.rs

@@ -2761,6 +2761,10 @@ pub const MCL_FUTURE: c_int = 0x0002;
 pub const XATTR_CREATE: c_int = 0x1;
 pub const XATTR_REPLACE: c_int = 0x2;
 
+// sys/xattr.h
+pub const XATTR_CREATE: ::c_int = 0x1;
+pub const XATTR_REPLACE: ::c_int = 0x2;
+
 // spawn.h
 pub const POSIX_SPAWN_USEVFORK: c_short = 64;
 pub const POSIX_SPAWN_SETSID: c_short = 128;