Snippets: Replace forward declaration by include

Avoid issues with QXmlStream* classes on Mac.

Reviewed-by: con

src/plugins/texteditor/snippets/snippetscollection.cpp

@@ -40,7 +40,6 @@
 #include <QtCore/QDir>
 #include <QtCore/QDebug>
 #include <QtCore/QXmlStreamReader>
-#include <QtCore/QXmlStreamWriter>
 #include <QtAlgorithms>
 
 #include <iterator>

src/plugins/texteditor/snippets/snippetscollection.h

@@ -35,8 +35,7 @@
 #include <QtCore/QVector>
 #include <QtCore/QStringList>
 #include <QtCore/QHash>
-
-QT_FORWARD_DECLARE_CLASS(QXmlStreamWriter)
+#include <QtCore/QXmlStreamWriter>
 
 namespace TextEditor {
 namespace Internal {