kbuild: fix perl usage in export_report.pl

Fixes some subtle perl coding bug observed
by Jan Engelhardt <jengelh@computergmbh.de>
This patch applies on top of Adrian's fix.

Signed-off-by: Ram Pai <linuxram@us.ibm.com>
Acked-by: Jan Engelhardt <jengelh@computergmbh.de>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>