Fix the windows build for real this time

* Mark gplugin_introspection_introspect_plugin and
gplugin_introspection_introspect_plugins as internal as this is a static
library and that is how they should be marked.
* Use G_MODULE_EXPORT to export the G_DEFINE_FINAL_TYPE for IntrospectableType.
* Clean up the gplugin-introspection/tests/meson.build file bit as it's
already shortcut in gplugin-introspection/meson.build.

Testing Done:
Compiled from a clean build on Windows and Linux using `ninja turtles` on both.

Reviewed at https://reviews.imfreedom.org/r/3030/