ENH: get most used extruder id

Jira: none

Signed-off-by: qing.zhang <qing.zhang@bambulab.com>
Change-Id: I9c498cefb07b3bdf89a17bfd75a3075c2294c934
(cherry picked from commit e0508a9e5136cec6ba69cc8a4d186079802f841a)

src/libslic3r/GCode/ToolOrdering.hpp

@@ -248,6 +248,7 @@ public:
 
     // should be called after doing reorder
     FilamentChangeStats get_filament_change_stats(FilamentChangeMode mode);
+    std::vector<int>    cal_most_used_extruder(const PrintConfig &config);
 
 private:
     void				initialize_layers(std::vector<coordf_t> &zs);