Add 'pkg-config' to list of Ubuntu packages

Created by: huntergoldstein

Currently running make will fail on

cargo run --manifest-path installer/redoxfs/Cargo.toml --release --bin redoxfs-mkfs build/filesystem.bin.partial

as this command will call into pkg-config --libs --cflags fuse (which clearly depends on pkg-config. This package did not get installed by bootstrap.sh inside a Xenial Vagrant Box; one figures its better to be explicit about this dependency.