Make MapFieldNames public (#2118) (#2134)

diff --git a/arrow/src/array/builder/mod.rs b/arrow/src/array/builder/mod.rs
index a5b7f1e..f97ff4c 100644
--- a/arrow/src/array/builder/mod.rs
+++ b/arrow/src/array/builder/mod.rs
@@ -52,7 +52,7 @@
 pub use generic_binary_builder::GenericBinaryBuilder;
 pub use generic_list_builder::GenericListBuilder;
 pub use generic_string_builder::GenericStringBuilder;
-pub use map_builder::MapBuilder;
+pub use map_builder::{MapBuilder, MapFieldNames};
 pub use primitive_builder::PrimitiveBuilder;
 pub use primitive_dictionary_builder::PrimitiveDictionaryBuilder;
 pub use string_dictionary_builder::StringDictionaryBuilder;