Fix error with function qualification. Refs #4399.

MSVC manages to pick the correct overload, however gcc needs some help.

Code/Mantid/Framework/PythonInterface/mantid/kernel/src/Exports/CompositeValidator.cpp

@@ -43,7 +43,7 @@ void export_CompositeValidator()
                                       (arg("validators"))
                                       ))
 
-    .def("add", &CompositeValidator::add, "Add another validator to the list")
+    .def("add", (void (CompositeValidator::*)(IValidator_sptr))&CompositeValidator::add, "Add another validator to the list")
   ;
 }