sparse-index.h: move declarations for sparse-index.c from cache.h

Note in particular that this reverses the decision made in 118a2e8bde0
("cache: move ensure_full_index() to cache.h", 2021-04-01).

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/sparse-index.h b/sparse-index.h
index 59a92d8..a16f3e6 100644
--- a/sparse-index.h
+++ b/sparse-index.h
@@ -37,4 +37,6 @@ struct pattern_list;
  */
 void expand_index(struct index_state *istate, struct pattern_list *pl);
 
+void ensure_full_index(struct index_state *istate);
+
 #endif