Remove transitive include for Detour (#1450)

Moves it to the 1 cpp file that uses it and locks down its header.

Still compiles and links.

dPhysics/dpWorld.cpp

@@ -81,7 +81,7 @@ void dpWorld::Shutdown() {
 }
 
 bool dpWorld::IsLoaded() {
-	return m_NavMesh->GetdtNavMesh() != nullptr;
+	return m_NavMesh->IsNavmeshLoaded();
 }
 
 void dpWorld::StepWorld(float deltaTime) {