MINOR: sample: add function for browsing samples.

This function is useful with the incoming lua functions.
diff --git a/include/proto/sample.h b/include/proto/sample.h
index 6b76662..19b9f02 100644
--- a/include/proto/sample.h
+++ b/include/proto/sample.h
@@ -40,6 +40,7 @@
 const char *sample_src_names(unsigned int use);
 const char *sample_ckp_names(unsigned int use);
 struct sample_fetch *find_sample_fetch(const char *kw, int len);
+struct sample_fetch *sample_fetch_getnext(struct sample_fetch *current, int *idx);
 int smp_resolve_args(struct proxy *p);
 int smp_expr_output_type(struct sample_expr *expr);
 int c_none(struct sample *smp);