Fix forward declaration living in the wrong namespace

cherry-pick of cd9200091d

Bounding_volumes/test/Bounding_volumes/min_sphere_test.cpp

@@ -33,10 +33,12 @@
 #include <CGAL/Random.h>
 #include <CGAL/Cartesian_d.h>
 #include <CGAL/Homogeneous_d.h>
-#include <sstream>
-#include <cassert>
 #include <CGAL/Min_sphere_annulus_d_traits_d.h>
 #include <CGAL/Min_sphere_d.h>
+#include <CGAL/Quotient.h>
+
+#include <cassert>
+#include <sstream>
 
 using namespace CGAL;
 

Number_types/include/CGAL/NT_converter.h

@@ -17,11 +17,10 @@
 #include <CGAL/number_type_config.h>
 #include <CGAL/number_utils.h>
 
-template <bool> class Interval_nt;
-
 namespace CGAL {
 
-// A number type converter usable as default, using the conversion operator.
+template <bool>
+class Interval_nt;
 
 template < class NT1, class NT2 >
 struct NT_converter