ipkg-build: suppress timestamps in gzip headers

When creating control.tar.gz, data.tar.gz and $package.ipk
tar compresses them with default gzip options.
This includes build timestamps in their gzip headers.
By setting the GZIP env variable to -n, gzip no longer embeds
them, which is a step towards reproducibility.

See also [1] for more detailed motivations.

[1]: https://wiki.debian.org/ReproducibleBuilds/About

Signed-off-by: Reiner Herrmann <reiner@reiner-h.de>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46026 3c298f89-4303-0410-b956-a3cf2f4a3e73