extension gir: Export 'libnemo-extension' to the gir

Fixes a missing package tag. It is 'libnemo-extension' instead of
'nemo' to be consistent with pkgconfig. (similar to
https://github.com/GNOME/nautilus/blob/master/libnautilus-extension/meson.build,
line 77)

Author: JPeisach

libnemo-extension/meson.build

@@ -69,6 +69,7 @@ gnome.generate_gir(nemo_extension_lib,
   sources: nemo_extension_sources + nemo_extension_headers,
   nsversion: '3.0',
   namespace: 'Nemo',
+  export_packages: 'libnemo-extension',
   includes: [ 'Gtk-3.0', 'Gio-2.0', 'GLib-2.0', ],
   include_directories: [ rootInclude, ],
   install_dir_typelib: typelibdir,