midx.h: add missing forward declarations (hdr-check)

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

midx.h

@@ -3,6 +3,9 @@
 
 #include "repository.h"
 
+struct object_id;
+struct pack_entry;
+
 struct multi_pack_index {
 	struct multi_pack_index *next;