Remove more boost leakage

cpp/src/arrow/python/helpers.cc

@@ -17,6 +17,7 @@
 
 #include "arrow/python/helpers.h"
 #include "arrow/python/common.h"
+#include "arrow/util/decimal.h"
 
 #include <arrow/api.h>
 

cpp/src/arrow/python/helpers.h

@@ -26,10 +26,13 @@
 
 #include "arrow/python/common.h"
 #include "arrow/type.h"
-#include "arrow/util/decimal.h"
 #include "arrow/util/visibility.h"
 
 namespace arrow {
+
+template <typename T>
+struct Decimal;
+
 namespace py {
 
 ARROW_EXPORT

cpp/src/arrow/python/python-test.cc

@@ -31,6 +31,8 @@
 #include "arrow/python/helpers.h"
 #include "arrow/python/pandas_convert.h"
 
+#include "arrow/util/decimal.h"
+
 namespace arrow {
 namespace py {
 

cpp/src/arrow/type.h

@@ -28,7 +28,6 @@
 
 #include "arrow/status.h"
 #include "arrow/type_fwd.h"
-#include "arrow/util/decimal.h"
 #include "arrow/util/macros.h"
 #include "arrow/util/visibility.h"
 #include "arrow/visitor.h"