include/starpu_perfmodel.h: only declare a type when needed

include/starpu_perfmodel.h

@@ -34,9 +34,6 @@ extern "C"
 #endif
 
 struct starpu_task;
-
-struct starpu_perfmodel_history_table;
-struct starpu_perfmodel_history_list;
 struct starpu_buffer_descr;
 
 /*
@@ -142,6 +139,8 @@ struct starpu_regression_model
 	unsigned nsample;
 };
 
+struct starpu_perfmodel_history_table;
+
 struct starpu_per_arch_perfmodel
 {
 	double (*cost_model)(struct starpu_buffer_descr *t) STARPU_DEPRECATED; /* returns expected duration in µs */