Packages: Used a more common name for pkg-config.

pkg-config package is named differently on supported rpm-based systems:
- Amazon Linux 2 has pkgconfig
- Fedora has pkgconf-pkg-config
- RHEL 7 has pkgconfig
- RHEL 8 and 9 have pkgconfig-pkg-config

What they share in common is they all provide 'pkgconfig', which we can
use in the spec file so we don't have to specify it per-OS.

pkg/rpm/unit.spec.in

@@ -46,7 +46,7 @@ Requires(preun):   systemd
 Requires(postun):  systemd
 
 BuildRequires: pcre2-devel
-BuildRequires: pkg-config
+BuildRequires: pkgconfig
 
 Provides: unit-r%{version}