qt: Fix building with Qt6

* lang/qt/src/util.cpp, lang/qt/src/util.h: Replace forward declaration
with include.
--

Based on patch by Laurent Montel

lang/qt/src/util.cpp

@@ -37,8 +37,6 @@
 
 #include "util.h"
 
-#include <QStringList>
-
 #include <key.h>
 
 #include <algorithm>

lang/qt/src/util.h

@@ -34,14 +34,14 @@
 #ifndef __QGPGME_UTIL_H__
 #define __QGPGME_UTIL_H__
 
+#include <QStringList>
+
 #include <gpgme.h>
 
 #include <sstream>
 #include <string>
 #include <vector>
 
-class QStringList;
-
 namespace GpgME
 {
 class Key;