csum-file.h: remove unnecessary inclusion of cache.h

With the change in the last commit to move several functions to
write-or-die.h, csum-file.h no longer needs to include cache.h.
However, removing that include forces several other C files, which
directly or indirectly dependend upon csum-file.h's inclusion of
cache.h, to now be more explicit about their dependencies.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pack.h

@@ -4,6 +4,8 @@
 #include "object.h"
 #include "csum-file.h"
 
+struct packed_git;
+struct pack_window;
 struct repository;
 
 /*