package: don't use .xz format for github

Fixes http://autobuild.buildroot.net/results/955/955ed1af28f6ff9912adddc0342d1f3ca65e5b4d/



Commit 62146ea3 (change package tarball compression to xz whenever possible)
changed a number of packages to use .xz format tarballs. Unfortunately this
doesn't work for packages from github, as it delivers gzip compressed tarballs
even when a .xz is requested.

Fix it by reverting this change for the packages using github.

Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>