src/conversion/ensight: Specify std::isnan

Resolves bug-report http://www.openfoam.org/mantisbt/view.php?id=2041

src/conversion/ensight/part/ensightPart.C

@@ -2,7 +2,7 @@
   =========                 |
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     |
-    \\  /    A nd           | Copyright (C) 2011-2015 OpenFOAM Foundation
+    \\  /    A nd           | Copyright (C) 2011-2016 OpenFOAM Foundation
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
 License
@@ -51,7 +51,7 @@ bool Foam::ensightPart::isFieldDefined(const List<scalar>& field) const
         {
             const label id = idList[i];
 
-            if (id >= field.size() || isnan(field[id]))
+            if (id >= field.size() || std::isnan(field[id]))
             {
                 return false;
             }