Added a missing const keyword.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%404237

dlib/image_transforms/image_pyramid.h

@@ -21,7 +21,7 @@ namespace dlib
         void operator() (
             const in_image_type& original,
             out_image_type& down
-        )
+        ) const
         {
             // make sure requires clause is not broken
             DLIB_ASSERT(original.nr() > 10 && original.nc() > 10 &&

dlib/image_transforms/image_pyramid_abstract.h

@@ -24,7 +24,7 @@ namespace dlib
         void operator() (
             const in_image_type& original,
             out_image_type& down
-        );
+        ) const;
         /*!
             requires
                 - original.nr() > 10