MINOR: htx: Add the function htx_move_blk_before()

The function htx_add_data_before() was removed because it was buggy. The
function htx_move_blk_before() may be used if necessary to do something
equivalent, except it just moves blocks. It doesn't handle the adding.
diff --git a/include/common/htx.h b/include/common/htx.h
index df75e33..e7d8b80 100644
--- a/include/common/htx.h
+++ b/include/common/htx.h
@@ -194,6 +194,7 @@
 struct htx_blk *htx_add_data_atonce(struct htx *htx, struct ist data);
 size_t htx_add_data(struct htx *htx, const struct ist data);
 struct htx_blk *htx_add_last_data(struct htx *htx, struct ist data);
+void htx_move_blk_before(struct htx *htx, struct htx_blk **blk, struct htx_blk **ref);
 
 int htx_reqline_to_h1(const struct htx_sl *sl, struct buffer *chk);
 int htx_stline_to_h1(const struct htx_sl *sl, struct buffer *chk);