Fix windows warning

...\qt5\include\qtcore\qcache.h(124): warning C4150: deletion of pointer to incomplete type 'QDomDocument'; no destructor called
.../server/qgsconfigcache.h(34): note: see declaration of 'QDomDocument'
...\qt5\include\qtcore\qcache.h(124): note: while compiling class template member function 'void QCache<QString,QDomDocument>::clear(void)'

src/server/qgsconfigcache.h

@@ -24,15 +24,14 @@
 #include <QFileSystemWatcher>
 #include <QMap>
 #include <QObject>
+#include <QDomDocument>
 
 #include "qgis_server.h"
 #include "qgswmsconfigparser.h"
 
 class QgsServerProjectParser;
 class QgsAccessControl;
 
-class QDomDocument;
-
 class SERVER_EXPORT QgsConfigCache : public QObject
 {
     Q_OBJECT