[doc] Include missing dependency install step

Summary

In response to #350 (closed) this MR just adds the missing install step for the dependency libprotobuf-dev to doc/build-unix-deb.md

Closes #350 (closed)

Test plan

• Get a fresh Ubuntu 18.04 install. Eg with: docker run -it ubuntu:18.04.
• Follow the initial build prep instructions at doc/build-unix-deb.md. Or just paste this condensed extract into the terminal (on the new machine):

apt-get update &&
apt-get -y install apt-transport-https ca-certificates gnupg software-properties-common wget  &&
wget -O - https://apt.kitware.com/keys/kitware-archive-latest.asc 2>/dev/null | apt-key add -  &&
apt-add-repository 'deb https://apt.kitware.com/ubuntu/ bionic main'  &&
apt-get update  &&
apt-get -y install build-essential cmake git libboost-chrono-dev libboost-filesystem-dev libboost-test-dev libboost-thread-dev libevent-dev libminiupnpc-dev libssl-dev libzmq3-dev help2man ninja-build python3 libdb-dev libdb++-dev libqrencode-dev protobuf-compiler qttools5-dev  &&
git clone https://gitlab.com/bitcoin-cash-node/bitcoin-cash-node.git  &&
cd bitcoin-cash-node/  &&
mkdir build  &&
cd build

• Try running cmake as is: cmake -GNinja .. and confirm that the error Could NOT find Protobuf appears.
• Ensure that the error does not appear when the GUI is disabled, with: cmake -GNinja .. -DBUILD_BITCOIN_QT=OFF
• Clear the build directory: cd ..; rm -r build; mkdir build; cd build
• Install the missing dependency with apt-get install libprotobuf-dev
• Rerun cmake -GNinja .. and confirm that it now completes without error.
• Might be worth checking that ninja succeeds too.