build: use build_by_default in cargo-build target

build_always_stale is deprecated and build_by_default can be
used now that we don't have build.rs.

Requires that we list our sources.

podcasts-data/src/meson.build

@@ -0,0 +1,19 @@
+data_sources = files(
+  'models/episode.rs',
+  'models/mod.rs',
+  'models/new_episode.rs',
+  'models/new_show.rs',
+  'models/new_source.rs',
+  'models/show.rs',
+  'models/source.rs',
+  'database.rs',
+  'dbqueries.rs',
+  'errors.rs',
+  'feed.rs',
+  'lib.rs',
+  'opml.rs',
+  'parser.rs',
+  'pipeline.rs',
+  'schema.rs',
+  'utils.rs',
+)