package/tzdata: do extract to get the license file

Back in 2014, about 6 years ago now, in my infinite wisdom, I
decided that extracting the tzdata source was not necessary for
the target variant, because we would be installing the files
generated by the host variant, in commit 7aad5daa (package/tzdata:
only compile the zoneinfo once).

However, that did not account for the fact that we would eventually
like to have the licensing information for tzdata, later added in
2019, in commit 60889ccd (package/tzdata: bump to version 2019b).

However, that last comit only added the license file to the host
variant, without explanations why that was so. It turns out that the
reason it was not added to the target variant is, probably, that he
source code for the target variant is not extracted, and thus saving
the license file fails.

But we really want the license file for what goes on into the target.

So, do extract the source code for the target variant, even if only to
get the license file.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Richard Braun <rbraun@sceen.net>
Cc: Martin Bark <martin@barkynet.com>
Cc: Christopher McCrory <chrismcc@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>