Only add GModule's include directory to GPlugin dependency

The GModule link arguments include `-Wl,--export-dynamic`, which breaks all
GObject Instropection, and is likely not a flag we want to force on all GPlugin
users.

Testing Done:
Compiled PurpleIRCv3, which used the gplugin-introspection, and it did not fail this time (with `ninja turtles`.)

Reviewed at https://reviews.imfreedom.org/r/2824/