fix broken #ifdefs in case openmp is not enabled

src/cluster/cluster.cpp

@@ -403,7 +403,6 @@ void cluster(const string& config_file, const string& data_file, const string& o
 
 	nvtxRangePush("Output concatenation");
 #endif
-#ifdef _OPENMP
 #pragma omp barrier
 	{
 	  // thread 0 already wrote on global files, skip it and take care of appending the others
@@ -457,7 +456,6 @@ void cluster(const string& config_file, const string& data_file, const string& o
 	  //   logger->log("done.\n", LOG_DEBG);
 	  // }
 	}
-#endif
 	// here go the code to append the files written in MPI processes > 0 to the ones on MPI process 0
 #ifdef MPI_VERSION
 	if (mpidata->mpirunning) {