[gi-docgen] Add missing licensing terms for the manifest

[Emmanuele Bassi <ebassi src gnome org>]

commit 9870ad7574cb218b3fc5cccae30c8987d8341d93
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Thu Apr 15 15:02:53 2021 +0100

    Add missing licensing terms for the manifest

 MANIFEST.in | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/MANIFEST.in b/MANIFEST.in
index 73e6a41..c420ea8 100644
--- a/MANIFEST.in
+++ b/MANIFEST.in
@@ -1,3 +1,6 @@
+# SPDX-FileCopyrightText: 2021  GNOME Foundation
+# SPDX-License-Identifier: CC0-1.0
+
 include README.md meson.build gi-docgen.pc.in
 include gi-docgen.py
 include .flake8