[release] MPITraits: Use std::uint16_t for bigunsignedint<k>

The storage of bigunsignedint was changed to std::uint16_t in commit
541b3dfc. This implements the same
change for the MPITraits<bigunsignedint<k>>

Note that this assumes that
  MPITraits<std::uint16_t>
is defined. This is the case as long as uint16_t maps to one of the
standard integer types (unsigned char/short/int/long).

Thanks to Markus for pointing this out.

Reference: https://dune-project.org/flyspray/index.php?do=details&task_id=1657#comment6393


(cherry picked from commit 586375b4)
Signed-off-by: Carsten Gräser <graeser@dune-project.org>

dune/common/parallel/mpitraits.hh

@@ -7,6 +7,7 @@
 #include <mpi.h>
 #endif
 
+#include <cstdint>
 #include <utility>
 
 namespace Dune
@@ -127,7 +128,7 @@ namespace Dune
     static inline MPI_Datatype getType()
     {
       if(datatype==MPI_DATATYPE_NULL) {
-        MPI_Type_contiguous(bigunsignedint<k>::n, MPITraits<unsigned short>::getType(),
+        MPI_Type_contiguous(bigunsignedint<k>::n, MPITraits<std::uint16_t>::getType(),
                             &vectortype);
         //MPI_Type_commit(&vectortype);
         bigunsignedint<k> data;