Trac #29449: pkgconfig should depend on pkgconf

As the summary says.

If I run "make numpy" on a system without its own pkg-config, the build
fails. If I install Sage's `pkgconf` first, it works. `numpy` depends on
`pkgconfig`, the Python interface to `pkgconf`, but `pkgconfig` really
only makes sense if `pkgconf` (or a system-wide equivalent) is
available.

URL: https://trac.sagemath.org/29449
Reported by: jhpalmieri
Ticket author(s): John Palmieri
Reviewer(s): Matthias Koeppe

build/pkgs/pkgconfig/dependencies

@@ -1,4 +1,4 @@
-$(PYTHON) | setuptools pip
+$(PYTHON) | setuptools pip pkgconf
 
 ----------
 All lines of this file are ignored except the first.