Remove warning for unused variable env in MPIInterface

It is not used, but I think it is needed for MPI to work properly

src/mpi_interface/MPI_Interface.hpp

@@ -75,7 +75,10 @@ public:
 
 namespace mpi_setup
 {
+#pragma GCC diagnostic ignored "-Wunused-variable"
 static mpi::environment* env = 0;            //!< The MPI environment
+#pragma GCC diagnostic pop
+
 extern std::shared_ptr<MPI_Interface> comm;  //!< The MPI communicator
 
 /**