FIX: Fix missing std::accumulate declaration

/run/build/BambuStudio/src/libslic3r/ExPolygon.cpp: In function ‘double Slic3r::get_expolygons_area(const ExPolygons&)’:
/run/build/BambuStudio/src/libslic3r/ExPolygon.cpp:437:17: error: ‘accumulate’ is not a member of ‘std’
  437 |     return std::accumulate(expolys.begin(), expolys.end(), (double)(0), [](double val, const ExPolygon& expoly) {
      |                 ^~~~~~~~~~

/run/build/BambuStudio/src/libslic3r/Polygon.cpp: In member function ‘bool Slic3r::Polygon::is_approx_circle(double, double, Slic3r::Point&, double&) const’:
/run/build/BambuStudio/src/libslic3r/Polygon.cpp:122:28: error: ‘accumulate’ is not a member of ‘std’
  122 |     double avg_dist = std::accumulate(distances.begin(), distances.end(), 0.0) / distances.size();
      |                            ^~~~~~~~~~

src/libslic3r/ExPolygon.cpp

@@ -9,6 +9,7 @@
 #include <algorithm>
 #include <cassert>
 #include <list>
+#include <numeric>
 
 namespace Slic3r {
 

src/libslic3r/Polygon.cpp

@@ -4,6 +4,8 @@
 #include "Polygon.hpp"
 #include "Polyline.hpp"
 
+#include <numeric>
+
 namespace Slic3r {
 
 double Polygon::length() const