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/mercurial/mercurialcommitwidget.cpp

@@ -69,7 +69,7 @@ public:
 private:
     enum State { Header, Comment, Other };
     const QTextCharFormat m_commentFormat;
-    const QRegExp m_keywordPattern;
+    QRegExp m_keywordPattern;
     const QChar m_hashChar;
 };