ipsec-tools: use a correct LIC_FILES_CHKSUM

The current LIC_FILES_CHKSUM checks the FSF copyright/license from the
*generated* Makefile.in, which has nothing at all to do with the licensing of
ipsec-tools. This is about as wrong as it's possible for LIC_FILES_CHKSUM to
be. This fixes it to use one of the headers from libipsec instead.

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>