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/landcover/landcover.yaml

@@ -1,5 +1,8 @@
 layer:
   id: "landcover"
+  requires:
+    tables:
+      - ne_50m_antarctic_ice_shelves_polys
   description: |
       Landcover is used to describe the physical material at the surface of the earth. At lower zoom levels this is
       from Natural Earth data for glaciers and ice shelves and at higher zoom levels the landcover is [implied by OSM tags](http://wiki.openstreetmap.org/wiki/Landcover). The most common use case for this layer