Compile after qt5/qtbase commit 75a0c7f9

Commit 75a0c7f9b52cde47f20fdc1b89e1264d60350848 in qt5/qtbase changed
some QRegExp methods to be non-const (they were previously const). This
change makes Qt Creator compile again.

Change-Id: Ibc98c678126c3b3189df7fcc043463b940951445
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>

src/plugins/texteditor/codeassist/basicproposalitemlistmodel.cpp

@@ -204,7 +204,7 @@ void BasicProposalItemListModel::filter(const QString &prefix)
 
         first = false;
     }
-    const QRegExp regExp(keyRegExp);
+    QRegExp regExp(keyRegExp);
 
     m_currentItems.clear();
     for (QList<BasicProposalItem *>::const_iterator it = m_originalItems.begin();