App: Single arg ctors must be explicit

src/App/DocumentObjectPyImp.cpp

@@ -476,7 +476,7 @@ PyObject* DocumentObjectPy::getSubObject(PyObject *args, PyObject *keywds)
         Py::Object obj;
         Py::Object pyObj;
         Base::Matrix4D mat;
-        SubInfo(const Base::Matrix4D &mat) : sobj(nullptr), mat(mat){}
+        explicit SubInfo(const Base::Matrix4D &mat) : sobj(nullptr), mat(mat){}
     };
 
     Base::Matrix4D mat;