android=arm64: Fix image build removing "debian-security" from sources.list
android=arm64: Fix image build removing "debian-security" from sources.list

This commit is an attempt to fix the following error:

  W: Failed to fetch http://cdn-fastly.deb.debian.org/debian-security/dists/jessie/updates/InRelease  Unable to find expected entry 'main/binary-arm64/Packages' in Release file (Wrong sources.list entry or malformed file)
  E: Some index files failed to download. They have been ignored, or old ones used instead.

Looking at "http://cdn-fastly.deb.debian.org/debian-security/dists/jessie/updates/InRelease", it
doesn't list "arm64", it only lists the following ones:

  Architectures: amd64 armel armhf i386

Considering that the build of June 15 was successful (see https://circleci.com/gh/dockcross/dockcross/1950),
the architecture was most likely dropped in a recent update. It could be
related to the fact security support for Debian Jessie ended on June 17 2018.
See https://en.wikipedia.org/wiki/Debian_version_history#Release_table

dockcross/dockcross,thewtex/cross-compilers,thewtex/cross-compilers,thewtex/cross-compilers,dockcross/dockcross,dockcross/dockcross,dockcross/dockcross,thewtex/cross-compilers