OpenSSL: Added source/old to PKG_SOURCE_URL

OpenSSL: Added source/old to PKG_SOURCE_URL

OpenSSL moves old versions of the library from
http://www.openssl.org/source/ to
http://www.openssl.org/source/old/$version/ breaking the old links.
That behavior breaks the OpenWRT-build every time OpenSSL releases
a new version.

This patch adds http://www.openssl.org/source/old/$version/ to the
PKG_SOURCE_URL of OpenSSL to avoid breaking the build whenever
OpenSSL releases a new version.

Signed-off-by: Kevin Kirsch <ranlvor@starletp9.de>
Reviewed-by: Alexander Dahl <post@lespocky.de>
Backport of r47860

SVN-Revision: 48845