Add missing copyright header to CMakeLists.txt

Bug: none
Change-Id: I72e708c562f7b3eadb6d7f2360d1c99fd31bd872
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/42949
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 7177635..79ce9b6 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,4 +1,18 @@
-cmake_minimum_required(VERSION 3.6.3)
+# Copyright 2020 The SwiftShader Authors. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+cmake_minimum_required(VERSION 3.6.3)
 
 set(CMAKE_CXX_STANDARD 14)