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.

Compiled from a clean build on Windows and Linux using ninja turtles on both.