attr.h: remove duplicate struct definition

struct index_state is declared more than once.

Signed-off-by: Elia Pinto <gitter.spiros@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

attr.h

@@ -121,7 +121,6 @@ struct git_attr;
 /* opaque structures used internally for attribute collection */
 struct all_attrs_item;
 struct attr_stack;
-struct index_state;
 
 /*
  * Given a string, return the gitattribute object that