Use int32 instead of uint32 for the buffer length for increased porta…

…bility.

export/planloader/planloader.h

@@ -8,7 +8,7 @@ using SerializedPlan = struct {
  // If set, contains a serialized ::substrait::proto::Plan object.
  char *buffer;
  // If buffer is set, this is the size of the buffer.
- uint32_t size;
+ int32_t size;
  // If null the buffer is valid, otherwise this points to an error message.
  char *errorMessage;
 };