libmemif: add private header size field

Change-Id: I09567c8ee9c92e91918840f80942a005069d9842
Signed-off-by: Jakub Grajciar <jgrajcia@cisco.com>
diff --git a/extras/libmemif/src/memif.h b/extras/libmemif/src/memif.h
index 38b5402..3fbce91 100644
--- a/extras/libmemif/src/memif.h
+++ b/extras/libmemif/src/memif.h
@@ -103,6 +103,7 @@
   memif_region_index_t region;
   memif_region_offset_t offset;
   memif_log2_ring_size_t log2_ring_size;
+  uint16_t private_hdr_size;	/* used for private metadata */
 } memif_msg_add_ring_t;
 
 typedef struct __attribute__ ((packed))