tree-wide: add missing header for glibc < 2.34 (#35756)

`unistd.h` gets into the dependency tree via
`/usr/include/bits/sigstksz.h`, which was added with glibc-2.34.

src/udev/udevd.c

@@ -5,6 +5,8 @@
  * Copyright © 2009 Scott James Remnant <scott@netsplit.com>
  */
 
+#include <unistd.h>
+
 #include "errno-util.h"
 #include "fd-util.h"
 #include "mkdir.h"

src/vmspawn/vmspawn-register.c

@@ -1,5 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
+#include <unistd.h>
+
 #include "sd-bus.h"
 #include "sd-id128.h"
 #include "sd-json.h"