QMimeXMLProvider: add missing out-of-line destructor

Fixes build with the latest GCC 7.

Change-Id: I4900a256ed1c6cb177d7f94d54e5b07c06ddad08
Task-number: QTBUG-56514
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

src/corelib/mimetypes/qmimeprovider.cpp

@@ -706,6 +706,10 @@ QMimeXMLProvider::QMimeXMLProvider(QMimeDatabasePrivate *db)
     initResources();
 }
 
+QMimeXMLProvider::~QMimeXMLProvider()
+{
+}
+
 bool QMimeXMLProvider::isValid()
 {
     return true;

src/corelib/mimetypes/qmimeprovider_p.h

@@ -132,6 +132,7 @@ class QMimeXMLProvider : public QMimeProviderBase
 {
 public:
     QMimeXMLProvider(QMimeDatabasePrivate *db);
+    ~QMimeXMLProvider();
 
     virtual bool isValid() Q_DECL_OVERRIDE;
     virtual QMimeType mimeTypeForName(const QString &name) Q_DECL_OVERRIDE;