MINOR: stream: export the function 'smp_create_src_stkctr'

Could be useful outside of this file.
diff --git a/include/proto/stream.h b/include/proto/stream.h
index 1360f28..02ac0f8 100644
--- a/include/proto/stream.h
+++ b/include/proto/stream.h
@@ -49,6 +49,7 @@
 struct task *process_stream(struct task *t);
 void default_srv_error(struct stream *s, struct stream_interface *si);
 struct stkctr *smp_fetch_sc_stkctr(struct session *sess, struct stream *strm, const struct arg *args, const char *kw);
+struct stkctr *smp_create_src_stkctr(struct session *sess, struct stream *strm, const struct arg *args, const char *kw);
 int parse_track_counters(char **args, int *arg,
 			 int section_type, struct proxy *curpx,
 			 struct track_ctr_prm *prm,