Fix tarball.rebuild on windows

Change-Id: I0a649548088428bd1a1fcedab76325fffa6b72a0
(cherry picked from commit 91bfacea9058b0a00feb58879c14666d31089c21)
Reviewed-on: https://gerrit.libreoffice.org/11475
Reviewed-by: Matúš Kukan <matus.kukan@collabora.com>
Tested-by: Matúš Kukan <matus.kukan@collabora.com>