kconfiglib: change SPDX-License-Identifier to ISC

Commit f219e013 (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>