contrib/rtos-helpers/FreeRTOS-openocd: change license to CC0-1.0

This file is intended to be included in any user's project that
plans to use OpenOCD awareness for FreeRTOS.
It is supposed to be distributed under a permissive license to not
impact the original license of the user's project.

Distribute it under Creative Commons Zero v1.0 Universal license.

Change-Id: I7ac90e8b742517bcafc08c00662625671c8159c4
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7995
Reviewed-by: Karl Palsson <karlp@tweak.au>
Reviewed-by: Paul Fertser <fercerpav@gmail.com>
Tested-by: jenkins

contrib/rtos-helpers/FreeRTOS-openocd.c

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: GPL-2.0-or-later
+// SPDX-License-Identifier: CC0-1.0
 
 /*
  * Since at least FreeRTOS V7.5.3 uxTopUsedPriority is no longer