Fix oversight in C++ initializer.

* The corresponding divergent behaviour was introduced in r3975.

tests/pio_write.c

@@ -28,7 +28,7 @@
 
 static const struct model_config default_setup
 #ifdef __cplusplus
-= { 12, 6, 3, 5, 5, CDI_FILETYPE_GRB, CDI_DATATYPE_PACK24,1,"grb"};
+  = { 12, 6, 3, 5, 5, FILETYPE_GRB, CDI_DATATYPE_PACK24, 1, 0, "grb"};
 #else
   = { .nlon = 12, .nts = 3, .nlat = 6, .nvars = 5,
       .filetype = CDI_FILETYPE_GRB, .datatype = CDI_DATATYPE_PACK24,