Simplify, add MSVC exports

LICENSE.txt

@@ -1877,7 +1877,7 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 ----------------------------------------------------------------------
 
-cpp/src/arrow/vendored/base64.h and base64.cpp have the following license
+cpp/src/arrow/vendored/base64.cpp has the following license
 
 ZLIB License
 

cpp/src/arrow/util/base64.h

@@ -17,4 +17,18 @@
 
 #pragma once
 
-#include "arrow/vendored/base64.h"  // IWYU pragma: export
+#include <string>
+
+#include "arrow/util/visibility.h"
+
+namespace arrow {
+namespace util {
+
+ARROW_EXPORT
+std::string base64_encode(unsigned char const*, unsigned int len);
+
+ARROW_EXPORT
+std::string base64_decode(std::string const& s);
+
+}  // namespace util
+}  // namespace arrow

cpp/src/arrow/vendored/base64.cpp

@@ -29,7 +29,7 @@
 
 */
 
-#include "base64.h"
+#include "arrow/util/base64.h"
 #include <iostream>
 
 namespace arrow {