MINOR: h1-htx: Update h1_copy_msg_data() to ease the traces in the mux-h1

This function now uses the address of the pointer to the htx message where the
copy must be performed. This way, when a zero-copy is performed, there is no
need to refresh the caller's htx message. It is a bit easier to do that way,
especially to add traces in the mux-h1.
diff --git a/include/proto/h1_htx.h b/include/proto/h1_htx.h
index da37215..51db02c 100644
--- a/include/proto/h1_htx.h
+++ b/include/proto/h1_htx.h
@@ -29,7 +29,7 @@
 
 int h1_parse_msg_hdrs(struct h1m *h1m, union h1_sl *h1sl, struct htx *dsthtx,
 		      struct buffer *srcbuf, size_t ofs, size_t max);
-int h1_parse_msg_data(struct h1m *h1m, struct htx *dsthtx,
+int h1_parse_msg_data(struct h1m *h1m, struct htx **dsthtx,
 		      struct buffer *srcbuf, size_t ofs, size_t max,
 		      struct buffer *htxbuf);
 int h1_parse_msg_tlrs(struct h1m *h1m, struct htx *dsthtx,