Add BuildConfiguration::disabledReason()

Not yet used.

Change-Id: I1f8763163e5f19e41d8e98517cf13d9bbb1e6ff8
Reviewed-on: http://codereview.qt.nokia.com/189
Reviewed-by: Christian Kandeler <christian.kandeler@nokia.com>

src/plugins/projectexplorer/buildconfiguration.cpp

@@ -279,6 +279,11 @@ bool BuildConfiguration::isEnabled() const
     return true;
 }
 
+QString BuildConfiguration::disabledReason() const
+{
+    return QString();
+}
+
 bool BuildConfigMacroExpander::resolveMacro(const QString &name, QString *ret)
 {
     if (name == QLatin1String("sourceDir")) {