pkgconfig module: fix incorrect Feature logging for uninstalled_variables
pkgconfig module: fix incorrect Feature logging for uninstalled_variables

The utility function that processes this for both 'variables' and
'uninstalled_variables' accepts a kwarg for the name of the argument,
but then hardcodes 'variables' in the warning message. This is
misleading.

mesonbuild/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson,pexip/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,pexip/meson