cmsis: fix license header in rtx_def

Upstream fix was merged ARM-software/CMSIS_5#1238. Included here as it will be overwritten once again updated

Author: 0xc0170

cmsis/CMSIS_5/CMSIS/RTOS2/RTX/Include/rtx_def.h

@@ -1,7 +1,19 @@
 /*
  * Copyright (c) 2021 Arm Limited. All rights reserved.
  *
- * This Software is licensed under an Arm proprietary license.
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * 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
+ *
+ * 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.
  *
  * -----------------------------------------------------------------------------
  *