Fixing compilation issues.

src/libslic3r/Polygon.hpp

@@ -21,7 +21,7 @@ public:
     Point& operator[](Points::size_type idx) { return this->points[idx]; }
     const Point& operator[](Points::size_type idx) const { return this->points[idx]; }
 
-    Polygon() {}
+    Polygon() = default;
     virtual ~Polygon() = default;
     explicit Polygon(const Points &points) : MultiPoint(points) {}
 	Polygon(std::initializer_list<Point> points) : MultiPoint(points) {}