Don't explicitly use shared_library on gplugin, gplugin-gtk3, and gplugin-gtk4
By just using the library function meson can choose to static link these into
another target.
Testing Done:
Did intial testing in the pidgin tree by hacking `gplugin/meson.build` and used `ldd` to verify that gplugin was staticly linked and that plugins still worked.
Reviewed at https://reviews.imfreedom.org/r/1346/