Install base-devel by default for the example Arch Linux build in build-unix.md

Summary: These instructions do not work out of the box on a fresh machine due to a missing compiler.

Test Plan:
I don't have an Arch machine, so I tried this in a docker container:

docker run -it archlinux:latest /bin/bash
pacman -S base-devel boost cmake git libevent ninja python
git clone https://github.com/Bitcoin-ABC/bitcoin-abc.git
cd bitcoin-abc
cmake -GNinja . -DBUILD_BITCOIN_WALLET=OFF -DBUILD_BITCOIN_QT=OFF -DENABLE_UPNP=OFF -DBUILD_BITCOIN_ZMQ=OFF -DUSE_JEMALLOC=OFF

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6931