indexer: correct missing includes

Docurium seems to choke on this header because it can't see both
git_indexer_progress & git_indexer_progress_cb, let's add the include.

include/git2/odb.h

@@ -11,6 +11,7 @@
 #include "types.h"
 #include "oid.h"
 #include "oidarray.h"
+#include "indexer.h"
 
 /**
  * @file git2/odb.h

include/git2/odb_backend.h

@@ -9,6 +9,7 @@
 
 #include "common.h"
 #include "types.h"
+#include "indexer.h"
 
 /**
  * @file git2/backend.h

include/git2/pack.h

@@ -9,6 +9,7 @@
 
 #include "common.h"
 #include "oid.h"
+#include "indexer.h"
 
 /**
  * @file git2/pack.h