Bad Interval_nt forward declaration in 5.4.x-branch

[mglisse]

cgal/Number_types/include/CGAL/NT_converter.h

Lines 20 to 22 in 017614c

	template <bool> class Interval_nt;
	namespace CGAL {

Interval_nt is forward declared, but in the wrong namespace. This was fixed in #6120. Without backporting the whole PR, I wonder if we could backport just this tiny piece to 5.4.x-branch, moving the declaration a few lines below.

#ifndef NOBUG
#include <CGAL/NT_converter.h>
#endif
#include <CGAL/Epeck_d.h>

int main(){
  boost::multiprecision::mpq_rational q = 42;
  CGAL::NT_converter<boost::multiprecision::mpq_rational,CGAL::Interval_nt<>> c;
  c(q);
}

fails to compile with 5.4 and would be fixed by such a change.