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

Note in particular that this reverses the decision made in 118a2e8bde
("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>

entry.c

@@ -6,6 +6,7 @@
 #include "gettext.h"
 #include "hex.h"
 #include "name-hash.h"
+#include "sparse-index.h"
 #include "streaming.h"
 #include "submodule.h"
 #include "symlinks.h"