Add missing copy assignment operator to QXmlSchema

Found by MSVC 2013 when.

Change-Id: I09c23afca4c9045523ae0722751fb27737b1b5f8
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

src/xmlpatterns/api/qxmlschema.cpp

@@ -93,6 +93,17 @@ QXmlSchema::QXmlSchema(const QXmlSchema &other)
 {
 }
 
+/*!
+  \since 5.4
+  Copies the resources of \a other into this instance, sharing
+  them to the extent possible.
+ */
+QXmlSchema &QXmlSchema::operator =(const QXmlSchema &other)
+{
+    d = other.d;
+    return *this;
+}
+
 /*!
   Destroys this QXmlSchema.
  */

src/xmlpatterns/api/qxmlschema.h

@@ -64,6 +64,7 @@ class Q_XMLPATTERNS_EXPORT QXmlSchema
     public:
         QXmlSchema();
         QXmlSchema(const QXmlSchema &other);
+        QXmlSchema &operator=(const QXmlSchema &other);
         ~QXmlSchema();
 
         bool load(const QUrl &source);