MINOR: h2: add new functions to produce an HTX message from an H2 response

The new function h2_prepare_htx_stsline() produces an HTX response message
from an H2 response presented as a list of header fields.
diff --git a/include/common/h2.h b/include/common/h2.h
index e916732..6c1475b 100644
--- a/include/common/h2.h
+++ b/include/common/h2.h
@@ -156,6 +156,7 @@
 
 int h2_make_h1_request(struct http_hdr *list, char *out, int osize, unsigned int *msgf);
 int h2_make_htx_request(struct http_hdr *list, struct htx *htx, unsigned int *msgf);
+int h2_make_htx_response(struct http_hdr *list, struct htx *htx, unsigned int *msgf);
 
 /*
  * Some helpful debugging functions.