setting DLL visibility flags when building on Windows

Windows linking issue
----------------------------

The visibility flags seem to fail in the non-library linking and building qtsoap.obj locally in your project mode.

Linking in library mode seems to work:

<pre>
CONFIG += qtsoap-uselib
include(../QtSOAP/src/qtsoap.pri)
</pre>

after building the DLLS in `QtSoap\build` beforehand.