Add ui/gcr.gresource.xml back into the build.

This fixes a regression in gcr-3, introduced during the migration from autotools to meson, where leaving out ui/gcr.gresources.xml from the build causes certificate import failures, because /org/gnome/gcr/ui/gcr-pkcs11-import-dialog.ui cannot be found.

Note that this patch only matters if you plan on making further gcr-3 releases. The files in question were moved into other projects as of gcr-4.

Fixes #86.