MINOR: sample: export 'sample_get_trash_chunk(void)'

This will be used on external fetch modules.
diff --git a/include/proto/sample.h b/include/proto/sample.h
index 471eb5f..47099b3 100644
--- a/include/proto/sample.h
+++ b/include/proto/sample.h
@@ -32,5 +32,6 @@
                                struct sample *p);
 void sample_register_fetches(struct sample_fetch_kw_list *psl);
 void sample_register_convs(struct sample_conv_kw_list *psl);
+struct chunk *sample_get_trash_chunk(void);
 
 #endif /* _PROTO_SAMPLE_H */