MINOR: htx: Add a field to set the memory used by headers in the HTX start-line

The field hdrs_bytes has been added in the structure htx_sl. It should be used
to set how many bytes are help by all headers, from the start-line to the
corresponding EOH block. it must be set to -1 if it is unknown.
diff --git a/include/common/htx.h b/include/common/htx.h
index 55d4931..ec99098 100644
--- a/include/common/htx.h
+++ b/include/common/htx.h
@@ -139,6 +139,9 @@
 
 	/* XXX 2 bytes unused */
 
+	int32_t hdrs_bytes;  /* Bytes held by all headers from this start-line
+			      * to the corresponding EOH. -1 if unknown */
+
 	unsigned int len[3]; /* length of differnt parts of the start-line */
 	char         l[0];
 };