MINOR: http: export the get_reason() function

This patch exports the get_reason() function.
diff --git a/include/proto/proto_http.h b/include/proto/proto_http.h
index dfa094a..7622cb5 100644
--- a/include/proto/proto_http.h
+++ b/include/proto/proto_http.h
@@ -113,6 +113,7 @@
 			  struct hdr_idx *idx, int occ,
 			  struct hdr_ctx *ctx, char **vptr, int *vlen);
 char *http_get_path(struct http_txn *txn);
+const char *get_reason(unsigned int status);
 
 struct http_txn *http_alloc_txn(struct stream *s);
 void http_init_txn(struct stream *s);