added QT_DECL_EXPORT/QT_DECL_IMPORT to allow building DLLs

Hi,
to be able to comply with the LGPL license, I need to be able to build the smtp client as a DLL on Windows. I added the necessary QT_DECL_* declarations for this. On Linux, they don't do anything, but on Windows they lead to the respective symbols being exported from the DLL.

Cheers