Allow calling init presizing mode from outside viewport.cpp.

src/tilehighlight_func.h

@@ -25,6 +25,7 @@ void ResetObjectToPlace();
 
 void VpSelectTilesWithMethod(int x, int y, ViewportPlaceMethod method);
 void VpStartPlaceSizing(TileIndex tile, ViewportPlaceMethod method, ViewportDragDropSelectionProcess process);
+void VpStartPreSizing();
 void VpSetPresizeRange(TileIndex from, TileIndex to);
 void VpSetPlaceSizingLimit(int limit);