[Pool] correctly add all headers for codegen

src/finn/custom_op/fpgadataflow/pool_batch.py

@@ -201,7 +201,7 @@ class Pool_Batch(HLSCustomOp):
 
     def global_includes(self):
         self.code_gen_dict["$GLOBALS$"] = ['#include "activations.hpp"']
-        self.code_gen_dict["$GLOBALS$"] = ['#include "maxpool.h"']
+        self.code_gen_dict["$GLOBALS$"] += ['#include "maxpool.h"']
         self.code_gen_dict["$GLOBALS$"] += ['#include "pool.hpp"']
 
     def defines(self, var):