memory_resource is in experimental

Index: rts/Sim/Misc/SmoothHeightMesh.h
--- rts/Sim/Misc/SmoothHeightMesh.h.orig
+++ rts/Sim/Misc/SmoothHeightMesh.h
@@ -3,7 +3,7 @@
 #ifndef SMOOTH_HEIGHT_MESH_H
 #define SMOOTH_HEIGHT_MESH_H
 
-#include <memory_resource>
+#include <experimental/memory_resource>
 #include <queue>
 #include <vector>
 
