MINOR: htx: Add a function to truncate all blocks after a specific offset

This function will be used to truncate all incoming data in a channel, keeping
outgoing ones.

This may be backported to 1.9.
diff --git a/include/common/htx.h b/include/common/htx.h
index 56a6c9b..94df4bb 100644
--- a/include/common/htx.h
+++ b/include/common/htx.h
@@ -170,6 +170,7 @@
 struct htx_blk *htx_defrag(struct htx *htx, struct htx_blk *blk);
 struct htx_blk *htx_add_blk(struct htx *htx, enum htx_blk_type type, uint32_t blksz);
 struct htx_blk *htx_remove_blk(struct htx *htx, struct htx_blk *blk);
+void htx_truncate(struct htx *htx, uint32_t offset);
 
 struct htx_blk *htx_replace_blk_value(struct htx *htx, struct htx_blk *blk,
 				      const struct ist old, const struct ist new);