Const'ify two static variables

Even though the variables are local, they were exposed as writable
references. Turning those variables into constants and propagating the
constness prevents accidental modifications.

Change-Id: If874ae4995fb95434ad28da6682c980f90666061
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/libs/extensionsystem/pluginspec_p.h

@@ -111,7 +111,7 @@ private:
     PluginSpec *q;
 
     bool reportError(const QString &err);
-    static QRegExp &versionRegExp();
+    static const QRegExp &versionRegExp();
 };
 
 } // namespace Internal