pkg-config.recipe: Make it a runtime dep on all platforms

It was supposed to be a runtime dep on Windows, but because we had the
same recipe twice inside the recipes/ dir, only the first one was
being parsed, so it was not a runtime dep even when we previously
wanted it to be.

By making it a runtime dep everywhere, we're ensuring that it is built
as the very first thing when building anything. This unfortunately
means that the recipe will be built twice: once in bootstrap and then
again in package.

Even more unfortunately, we actually need that because we need to ship
the built pkg-config in our binary releases. For instance, because
people that consume the built binaries need a pkg-config (because
macOS and Windows do not ship it) or they need a pkg-config that
defaults to `--define-prefix` (on Linux).

Fixes gstreamer/cerbero#346
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