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/
syntax: glob
.*.swp
*.debhelper
*.orig
*.pyc
*.ui~
\#*.ui\#
syntax: regexp
^build.*\/
^packaging\/debian\/tmp\/
^packaging\/debian\/(.+\.)?(substvars|debhelper\.log)$
^packaging\/debian\/build.*\/
^packaging\/debian\/libgplugin0?(-.+)?\/
^packaging\/debian\/gir1.2-gplugin-0.0\/
^packaging\/debian\/files$
^po\/.*\.pot$
^obj-x86_64-linux-gnu\/
^debian\/?$
^subprojects\/.+\/