fix: indicate dependency on pkgconf in ubuntu docs (#6646)

This PR changes the ubuntu docs to indicate that Lean now requires
pkgconf to build.

This is a companion to #6643, but I can't push directly to that branch.

doc/make/ubuntu.md

@@ -8,5 +8,5 @@ follow the [generic build instructions](index.md).
 ## Basic packages
 
 ```bash
-sudo apt-get install git libgmp-dev libuv1-dev cmake ccache clang
+sudo apt-get install git libgmp-dev libuv1-dev cmake ccache clang pkgconf
 ```