[cleanup] Canonicalize the single-inclusion macro

dune/solvers/norms/pnorm.hh

 // -*- tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
 // vi: set et ts=8 sw=4 sts=4:
-#ifndef __PNORM__HH__
-#define __PNORM__HH__
+#ifndef DUNE_SOLVERS_PNORM_HH
+#define DUNE_SOLVERS_PNORM_HH
 
 #include <cmath>