more missing includes (#6398)

Here are more missing header files recently identified by building
without precompiled header support.

src/libslic3r/Arachne/SkeletalTrapezoidationGraph.cpp

@@ -2,6 +2,7 @@
 //CuraEngine is released under the terms of the AGPLv3 or higher.
 
 #include "SkeletalTrapezoidationGraph.hpp"
+#include "../Line.hpp"
 #include <ankerl/unordered_dense.h>