polarssl: fix download URL

Switch to a non-redirect download URL to fix:
http://autobuild.buildroot.net/results/ec3/ec340fffa6eebb18a0746097419359b44c557a90/


Thanks go to Paul Bakker for the quick response!

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