kconfiglib: change SPDX-License-Identifier to ISC

Commit f219e01311 (tools: Import Kconfiglib)
added SPDX GPL-2.0+ to this library by mistake.
It should be ISC.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Ulf Magnusson <ulfalizer@gmail.com>

tools/buildman/kconfiglib.py

@@ -1,5 +1,5 @@
 #
-# SPDX-License-Identifier:	GPL-2.0+
+# SPDX-License-Identifier:	ISC
 #
 # Author: Ulf Magnusson
 #   https://github.com/ulfalizer/Kconfiglib