Fix an issue where meson would fail if building without gtk3 support

Testing Done:
Ran `meson` (with no options) and verified that `ninja doc` built both `gplugin-doc` and `gplugin-gtk-doc`.

Ran `meson -Dgtk3=false` and verified that `ninja doc` just built `gplugin-doc`

Reviewed at https://reviews.imfreedom.org/r/629/