Remove unnecessary cast from WinHttpSetStatusCallback() call

This doesn't seem to be needed, our callback has the correct signature.

If it's required for some non-MSVC compilers (e.g. MinGW with old SDK),
it would be better to use the cast only conditionally to at least keep
the MSVC build type-safe.

src/msw/webrequest_winhttp.cpp

@@ -291,7 +291,7 @@ void wxWebRequestWinHTTP::Start()
 
     // Register callback
     if ( ::WinHttpSetStatusCallback(m_request,
-        (WINHTTP_STATUS_CALLBACK)wxRequestStatusCallback,
+        wxRequestStatusCallback,
         WINHTTP_CALLBACK_FLAG_READ_COMPLETE |
         WINHTTP_CALLBACK_FLAG_WRITE_COMPLETE |
         WINHTTP_CALLBACK_FLAG_SENDREQUEST_COMPLETE |