slic3r: Fix std::future declaration

src/slic3r/GUI/StepMeshDialog.hpp:40:10: error: ‘future’ in namespace ‘std’ does not name a template type
   40 |     std::future<unsigned int> task;
      |          ^~~~~~
src/slic3r/GUI/StepMeshDialog.hpp:8:1: note: ‘std::future’ is defined in header ‘<future>’; did you forget to ‘#include <future>’?
    7 | #include "Widgets/Button.hpp"
  +++ |+#include <future>
    8 | class Button;

src/slic3r/GUI/StepMeshDialog.hpp

@@ -1,6 +1,7 @@
 #ifndef _STEP_MESH_DIALOG_H_
 #define _STEP_MESH_DIALOG_H_
 
+#include <future>
 #include <thread>
 #include "GUI_Utils.hpp"
 #include "libslic3r/Format/STEP.hpp"
@@ -44,4 +45,4 @@ private:
     void stop_task();
 };
 
-#endif  // _STEP_MESH_DIALOG_H_
+#endif  // _STEP_MESH_DIALOG_H_