uclibc: snapshots are bz2

Snapshot tarballs are only in bz2 format, fixes:
http://autobuild.buildroot.net/results/b02/b021b296309d01405ad45344055863a5761abd70/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>