Add build dependency on pkg-config from pkgconf

The absence of this build dependency made the check for whether
pkg-config works fail (because it was not there) at the ./configure
stage, making RbConfig::CONFIG["PKG_CONFIG"] empty, and therefore broke
the usage of pkg_config() in extconf.rb scripts.

This was noticed by Lucas Kanashiro (thanks!) in Ubuntu while rebuilding
all Ruby packages to add ruby3.1 support, where ruby-augeas and
ruby-libvirt failed to build.

Gbp-Dch: full

Author: terceiro

debian/control

@@ -15,6 +15,7 @@ Build-Depends: bison,
                libyaml-dev,
                netbase <!nocheck>,
                openssl <!nocheck>,
+               pkg-config (>= 1.8.0-7~),
                procps <!nocheck>,
                ruby3.1:native <cross>,
                rubygems-integration (>= 1.6) <!nocheck>,