[core] add explicit std::hash<mbgl::style::ClassID> implementation to appease GCC

src/mbgl/style/class_dictionary.hpp

@@ -3,6 +3,7 @@
 #include <cstdint>
 #include <string>
 #include <unordered_map>
+#include <functional>
 
 namespace mbgl {
 namespace style {
@@ -34,3 +35,19 @@ private:
 
 } // namespace style
 } // namespace mbgl
+
+namespace std {
+
+// Explicitly define std::hash<style::ClassID> because GCC doesn't automatically use std::hash<> of
+// the underlying enum type.
+
+template <>
+struct hash<mbgl::style::ClassID> {
+public:
+    size_t operator()(const mbgl::style::ClassID id) const {
+        using T = std::underlying_type_t<mbgl::style::ClassID>;
+        return std::hash<T>()(static_cast<T>(id));
+    }
+};
+
+} // namespace std