Add curl to Gitian setup instrustions

curl is required to fetch dependencies

[ci skip]

doc/gitian-building.md

@@ -252,7 +252,7 @@ First we need to log in as `root` to set up dependencies and make sure that our
 user can use the sudo command. Type/paste the following in the terminal:
 
 ```bash
-apt-get install git ruby sudo apt-cacher-ng qemu-utils debootstrap lxc python-cheetah parted kpartx bridge-utils make ubuntu-archive-keyring
+apt-get install git ruby sudo apt-cacher-ng qemu-utils debootstrap lxc python-cheetah parted kpartx bridge-utils make ubuntu-archive-keyring curl
 adduser debian sudo
 ```