Fix another build error on Windows.

Author: c-lipka

source/core/math/spline.h

@@ -16,7 +16,7 @@
 /// @parblock
 ///
 /// Persistence of Vision Ray Tracer ('POV-Ray') version 3.8.
-/// Copyright 1991-2017 Persistence of Vision Raytracer Pty. Ltd.
+/// Copyright 1991-2018 Persistence of Vision Raytracer Pty. Ltd.
 ///
 /// POV-Ray is free software: you can redistribute it and/or modify
 /// it under the terms of the GNU Affero General Public License as
@@ -47,18 +47,20 @@
 // Module config header file must be the first file included within POV-Ray unit header files
 #include "core/configcore.h"
 
+#include "base/types.h"
+
 namespace pov
 {
 
+using namespace pov_base;
+
 //##############################################################################
 ///
 /// @defgroup PovCoreMathSpline Splines
 /// @ingroup PovCoreMath
 ///
 /// @{
 
-typedef DBL EXPRESS[5];
-
 struct SplineEntry
 {
     DBL par;      // Parameter