DLL export consistently when using __declspec(dllexport)

[kiyolee]

The 2 entry points (adler32_combine64() and crc32_combine64()) are in the export .def file. But if builing DLL without using the .def file, __declspec(dllexport) (behind ZEXTERN/ZEXPORT) is not in effect for the 2 entry points. So the final export list using __declspec(dllexport) and the .def file are differ by the 2 entry points. This fix is to keep the 2 methods consistent.