Make _ucount int instead of size_t in mpi_image_info

The struct stores offsets in a struct so there is no reason why this should be
size_t. The compiler warns about a comparison `<0` for size_t in `ompi_field_offset`.

Signed-off-by: Joseph Schuchart <joseph.schuchart@stonybrook.edu>

Author: devreal

ompi/debuggers/ompi_common_dll_defs.h

@@ -247,7 +247,7 @@ typedef struct
             int MPI_TAG;
             int MPI_ERROR;
             int _cancelled;
-            size_t _ucount;
+            int _ucount;
         } offset;
     } ompi_status_public_t;
     /* datatype structure */