IVGCVSW-7024 Add missing license info for reuse lint

Signed-off-by: Jim Flynn <jim.flynn@arm.com>
Change-Id: I97dee6982e0a7be01c13e9e803c0997547a39ff1
diff --git a/docs/stylesheet.css b/docs/stylesheet.css
index ea7f4dc..28fdba0 100644
--- a/docs/stylesheet.css
+++ b/docs/stylesheet.css
@@ -1,5 +1,6 @@
 /* Copyright (c) 2020 ARM Limited. */
-/* Changes to tabs.css */
+/* SPDX-License-Identifier: MIT    */
+/* Changes to tabs.css             */
 
 .tabs, .tabs2, .tabs3 {
     position: relative;