Add missing SPDX License Identifier for gpu documentation index file.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 Documentation/gpu/index.rst | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/Documentation/gpu/index.rst b/Documentation/gpu/index.rst
index eee5996acf2c..9a6c7831d7fe 100644
--- a/Documentation/gpu/index.rst
+++ b/Documentation/gpu/index.rst
@@ -1,3 +1,5 @@
+.. SPDX-License-Identifier: MIT
+
 ============================
 GPU Driver Developer's Guide
 ============================
-- 
2.25.1