Add relevant header for 'assert'.

Found out it wouldn't compile in debug mode anymore otherwise -- at least on Windows.

done as part of CURA-12490 -- but mostly unrelated to it

Author: rburema

include/SkeletalTrapezoidationEdge.h

@@ -4,6 +4,7 @@
 #ifndef SKELETAL_TRAPEZOIDATION_EDGE_H
 #define SKELETAL_TRAPEZOIDATION_EDGE_H
 
+#include <cassert>
 #include <list>
 #include <memory> // smart pointers
 #include <vector>

src/geometry/LinesSet.cpp

@@ -3,6 +3,7 @@
 
 #include "geometry/LinesSet.h"
 
+#include <cassert>
 #include <numeric>
 
 #include "geometry/ClosedLinesSet.h"

src/utils/ListPolyIt.cpp

@@ -3,6 +3,7 @@
 
 #include "utils/ListPolyIt.h"
 
+#include <cassert>
 #include <cmath> // isfinite
 #include <sstream> // ostream