Force creating import library for "adv" DLL under MSW

No library was created as no symbols were exported by the DLL any more,
so add a dummy symbol just to force the import library creation -- this
is important to let the projects referencing it to continue to link,
even if it's now useless.

src/common/dummy.cpp

@@ -26,3 +26,13 @@
 #ifdef __WINDOWS__
     #include "wx/msw/msvcrt.h"
 #endif
+
+#ifdef WXMAKINGDLL_ADV
+
+// This is a hack required to create an import library for the "adv" DLL which
+// doesn't contain anything any more and is preserved only for compatibility.
+extern WXEXPORT void wxAdvLibraryIsObsoleteDontUse()
+{
+}
+
+#endif // WXMAKINGDLL_ADV