fix build failures on mingw caused by name clash

In mingw.org, basetyps.h contains

    #define interface _COM_interface

which clashes with function parameter names in qdbusmessage.h.
Although there is no clash when building qtbase itself, the clash
makes building qttools 5.0.1 fail. Presumably this could also affect
other applications.

Taken from 2cc9a9a51d6742708b1ea41c7338755e2a0ee9e9 which solves the
same problem in another header.

Change-Id: I802d5c673b544fb3a17e9273030876928faa5c46
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/dbus/qdbusmessage.h

@@ -49,6 +49,10 @@
 
 #ifndef QT_NO_DBUS
 
+#if defined(Q_OS_WIN) && defined(interface)
+#  undef interface
+#endif
+
 QT_BEGIN_HEADER
 
 QT_BEGIN_NAMESPACE