Add SPDX license identifiers to source files under the LGPL

This follows what the kernel is doing, c.f.
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=5fd54ace4721fc5ce2bb5aef6318fcf17f421460.

src/nspawn/nspawn-patch-uid.h

@@ -1,5 +1,4 @@
-#pragma once
-
+/* SPDX-License-Identifier: LGPL-2.1+ */
 /***
   This file is part of systemd.
 
@@ -19,6 +18,8 @@
   along with systemd; If not, see <http://www.gnu.org/licenses/>.
 ***/
 
+#pragma once
+
 #include <sys/types.h>
 
 int fd_patch_uid(int fd, uid_t shift, uid_t range);