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/ac-power/ac-power.c

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
 /***
   This file is part of systemd.
 

src/activate/activate.c

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
 /***
   This file is part of systemd.
 

src/analyze/analyze-verify.c

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
 /***
   This file is part of systemd.
 

src/analyze/analyze-verify.h

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
 #pragma once
 
 /***

src/analyze/analyze.c

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
 /***
   This file is part of systemd.
 

src/ask-password/ask-password.c

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
 /***
   This file is part of systemd.
 

src/backlight/backlight.c

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
 /***
   This file is part of systemd.
 

src/basic/af-list.c

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
 /***
   This file is part of systemd.
 

src/basic/af-list.h

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
 #pragma once
 
 /***

src/basic/alloc-util.c

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
 /***
   This file is part of systemd.
 

src/basic/alloc-util.h

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
 #pragma once
 
 /***

src/basic/architecture.c

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
 /***
   This file is part of systemd.
 

src/basic/architecture.h

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
 #pragma once
 
 /***

src/basic/arphrd-list.c

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
 /***
   This file is part of systemd.
 

src/basic/arphrd-list.h

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
 #pragma once
 
 /***

src/basic/async.c

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
 /***
   This file is part of systemd.
 

src/basic/async.h

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
 #pragma once
 
 /***

src/basic/audit-util.c

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
 /***
   This file is part of systemd.
 

src/basic/audit-util.h

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
 #pragma once
 
 /***

src/basic/barrier.c

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
 /***
   This file is part of systemd.