Update combination.h

src/ibea/combination.h

 #ifndef COMBINATION_H
 #define COMBINATION_H
-
-Population SBX20(Population pop);
-
+#include "../coco/coco.h"
+#include "population.h"
+double simple_combination(double parent, const double eta, coco_random_state_t *random_generator);
+void combinate(Population population, Population & pop, coco_random_state_t *random_generator);
 #endif
+