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/1.0.2/ 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/1.0.2/ to the PKG_SOURCE_URL
of OpenSSL to avoid breaking the build whenever OpenSSL releases a new
version.