missing-syscalls: add license header in the version-controlled generated file

If the file was always generated on the fly, the header would be pointless.
But since we distribute it, it should be there. C.f.
a0e150b2f4.
This was forgotten in 35b42e5600.

src/basic/missing_syscalls.py

@@ -1,4 +1,6 @@
 #!/usr/bin/env python3
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
 import sys
 import functools
 
@@ -99,7 +101,10 @@ def print_syscall_def(syscall, tables, out):
           file=out)
 
 def print_syscall_defs(syscalls, tables, out):
-    print('/* This file is generated. Do not edit! */\n', file=out)
+    print('''\
+/* SPDX-License-Identifier: LGPL-2.1-or-later
+ * This file is generated. Do not edit! */
+''' , file=out)
     for syscall in syscalls:
         print_syscall_def(syscall, tables, out)