d/control: build-depend on source-highlight

d/control: build-depend on source-highlight

This ensures that code blocks with [source,<lang>] blocks get
rendered correctly in HTML output.

While we only have one such block, and we could remove the
source highlighting tag completely here we use this in pve-docs and
also have the dependency there correctly, so it makes sense to make
share this here, else a copied text from pve-docs may be incompatible
here

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>