[big unsigned int] MPITraits is a struct and no class

(calms down clang warning)

[[Imported from SVN: r7113]]

dune/common/bigunsignedint.hh

@@ -146,7 +146,7 @@ namespace Dune
   private:
     unsigned short digit[n];
 #if HAVE_MPI
-    friend class MPITraits<bigunsignedint<k> >;
+    friend struct MPITraits<bigunsignedint<k> >;
 #endif
     inline void assign(std::size_t x);