qt: Add export macro for QDebug operator

* lang/qt/src/debug.h: Add export macro.

--
This is public API so it should be exported. This did not
resolve a link issue for Windows that I've experienced
when building. But this might be to some uncleanlyness
in my build environment.

lang/qt/src/debug.h

@@ -34,6 +34,8 @@
 #ifndef QGPGME_DEBUG_H
 #define QGPGME_DEBUG_H
 
+#include "qgpgme_export.h"
+
 namespace GpgME
 {
 class Error;
@@ -41,6 +43,6 @@ class Error;
 
 class QDebug;
 
-QDebug operator<<(QDebug debug, const GpgME::Error &err);
+QGPGME_EXPORT QDebug operator<<(QDebug debug, const GpgME::Error &err);
 
 #endif // QGPGME_DEBUG_H