gplugin/gplugin

Fix some issues with the gtk3 and gtk4 includes when not used in a subproject

Testing Done:
Ran the unit tests and gplugin-gtk4-viewer and verified no issues.

Bugs closed: GPLUGIN-145

Reviewed at https://reviews.imfreedom.org/r/1469/
[wrap-git]
directory=gi-docgen
url=https://gitlab.gnome.org/GNOME/gi-docgen.git
push-url=ssh://git@gitlab.gnome.org:GNOME/gi-docgen.git
revision=main
depth=1