gplugin/gplugin

Update the POTFILES for the gtk->gtk3 rename

2021-09-30, Gary Kramlich
54627f4294b7
Update the POTFILES for the gtk->gtk3 rename

Testing Done:
ran `ninja gplugin-pot`

Reviewed at https://reviews.imfreedom.org/r/967/
if get_option('rs')
add_languages('rust')
subdir('tests')
endif # rs