SPDX: use the Beerware identifier.

cpukit/score/src/kern_tc.c

@@ -16,6 +16,8 @@
  */
 
 /*-
+ * SPDX-License-Identifier: Beerware
+ *
  * ----------------------------------------------------------------------------
  * "THE BEER-WARE LICENSE" (Revision 42):
  * <phk@FreeBSD.ORG> wrote this file.  As long as you retain this notice you