[transit] Replace inline static kSubwayLayerTypes with extern + cpp.

transit/CMakeLists.txt

@@ -11,6 +11,7 @@ set(
   experimental/transit_types_experimental.cpp
   experimental/transit_types_experimental.hpp
   transit_display_info.hpp
+  transit_entities.cpp
   transit_entities.hpp
   transit_graph_data.cpp
   transit_graph_data.hpp

transit/transit_entities.cpp

@@ -0,0 +1,7 @@
+#include "transit/transit_entities.hpp"
+
+namespace transit
+{
+std::unordered_set<std::string> const kSubwayLayerTypes{"subway", "train", "light_rail",
+                                                        "monorail"};
+}  // namespace transit

transit/transit_entities.hpp

@@ -43,8 +43,7 @@ inline std::string const kTransfersFile = "transfers" + kTransitFileExtension;
 inline std::string const kGatesFile = "gates" + kTransitFileExtension;
 
 // Route types shown on the subway layer.
-static std::unordered_set<std::string> const kSubwayLayerTypes{"subway", "train", "light_rail",
-                                                               "monorail"};
+extern std::unordered_set<std::string> const kSubwayLayerTypes;
 
 // Unique id for transit entities. It is generated by gtfs_converter and is persistent between
 // re-runs. Generated based on the unique string hash of the GTFS entity. Lies in the interval