Add requires.tables to <layer>.yaml (#1236)

**NOTE** this can only be merged after the next tools version is released.

Added required Postgres tables to the `<layer>.yaml` definition.
Close: #1220

PR of tools: https://github.com/openmaptiles/openmaptiles-tools/pull/370

layers/transportation/transportation.yaml

@@ -1,5 +1,8 @@
 layer:
   id: "transportation"
+  requires:
+    tables:
+      - ne_10m_admin_0_countries
   description: |
       **transportation** contains roads, railways, aerial ways, and shipping
        lines.