Remove unused variable warning when MPI is disabled.

testing/adios2/engine/bp/TestBPNoXMLRecovery.cpp

@@ -9,8 +9,8 @@
 int main(int argc, char *argv[])
 {
     int rank = 0;
-    int size = 1;
 #ifdef ADIOS2_HAVE_MPI
+    int size = 1;
     MPI_Init(&argc, &argv);
     MPI_Comm_rank(MPI_COMM_WORLD, &rank);
     MPI_Comm_size(MPI_COMM_WORLD, &size);