Buildfix - define header function inline

would otherwise cause a compiler error on multiple usages

MantleAPI/include/MantleAPI/Traffic/entity_helper.h

@@ -21,7 +21,7 @@
 
 namespace mantle_api
 {
-void SetSpeed(mantle_api::IEntity* entity, double velocity)
+inline void SetSpeed(mantle_api::IEntity* entity, double velocity)
 {
   auto orientation = entity->GetOrientation();