Leave Rust sources to devel/cargo module

Index: meson.build
--- meson.build.orig
+++ meson.build
@@ -10,6 +10,7 @@ subdir('data')
 subdir('po')
 subdir('src')
 
+subdir_done()
 flatpak_cargo_generator = find_program(meson.project_source_root() / 'build-aux/flatpak-cargo-generator.py')
 
 cargo_sources = custom_target(
