Ignore unsused typedef.

GCC warns about it.

dune/common/test/nullptr-test.cc

@@ -5,10 +5,11 @@
 #endif
 
 #include <dune/common/nullptr.hh>
+#include <dune/common/unused.hh>
 
 void basic_tests()
 {
-  typedef Dune::nullptr_t NULLPTR_T;
+  typedef Dune::nullptr_t NULLPTR_T DUNE_UNUSED;
   char* ch = nullptr;    // ch has the null pointer value
   char* ch2 = 0;         // ch2 has the null pointer value
 #ifdef FAIL