gplugin/gplugin

Make sure gtk4 was found before trying to build vala bindings against it

This error only showed up when gtk4 was enabled but not found.

Testing Done:
Built on haiku that doesn't have gtk without setting `-Dgtk4=false`.
Built on debian testing normally.

Reviewed at https://reviews.imfreedom.org/r/3069/
if get_option('nls')
i18n.gettext(GETTEXT_PACKAGE, preset: 'glib')
else
summary('translations',
'You have disabled installation of translations which means ' +
'English will be the only available language.',
section : 'Warnings')
endif