Missing 'asciidoc' dependency for i386 PTTBB build

The i386 PTTBB build fails on dependency '/usr/bin/a2x' when performing 'make gnulinux-i686' as per the instructions in bundle-gnulinux.txt [1].

This can be resolved by adding 'asciidoc' to the list of packages to install in the 'sudo apt-get install..' step.

[1] https://gitweb.torproject.org/pluggable-transports/bundle.git/blob/HEAD:/bundle-gnulinux.txt

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