gplugin/gplugin

Add gplugin-gtk4-viewer and its man page to the rpm spec file

Testing Done:
Ran the `fedora-34-amd64` convey plan successfully.

Reviewed at https://reviews.imfreedom.org/r/1097/
if get_option('rs')
e = executable('test-rust-loading', 'test-rust-loading.c',
include_directories : include_directories('.'),
c_args: [
'-DRUST_PLUGIN_DIR="@0@/plugins"'.format(meson.current_build_dir()),
],
link_with : gplugin_loader_tests,
dependencies : [GLIB, GOBJECT, gplugin_dep])
test('Rust loading', e)
subdir('plugins')
endif # rs