Add missing declaration

Author: Rafał Hibner

cpp/src/arrow/acero/exec_plan_internal.h

@@ -33,5 +33,6 @@ void RegisterSinkNode(ExecFactoryRegistry*);
 void RegisterHashJoinNode(ExecFactoryRegistry*);
 void RegisterAsofJoinNode(ExecFactoryRegistry*);
 void RegisterSortedMergeNode(ExecFactoryRegistry*);
+void RegisterPipeNodes(ExecFactoryRegistry*);
 
 }  // namespace arrow::acero::internal

cpp/src/arrow/acero/pipe_node.cc

@@ -18,6 +18,7 @@
 #include "arrow/acero/pipe_node.h"
 #include <iostream>
 #include "arrow/acero/exec_plan.h"
+#include "arrow/acero/exec_plan_internal.h"
 #include "arrow/acero/map_node.h"
 #include "arrow/acero/options.h"
 #include "arrow/acero/query_context.h"