QMimeProvider: fix a clazy warning

Polymorphic class QMimeXMLProvider is copyable. Potential slicing.
[-Wclazy-copyable-polymorphic].

Change-Id: I1eff1389140ce9426db32a37df89935a7b3db3a5
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

src/corelib/mimetypes/qmimeprovider_p.h

@@ -31,6 +31,8 @@ class QMimeMagicRuleMatcher;
 
 class QMimeProviderBase
 {
+    Q_DISABLE_COPY(QMimeProviderBase)
+
 public:
     QMimeProviderBase(QMimeDatabasePrivate *db, const QString &directory);
     virtual ~QMimeProviderBase() {}