write-or-die.h: move declarations for write-or-die.c functions from cache.h

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/refs/files-backend.c b/refs/files-backend.c
index d2b8925..e6a6971 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -17,6 +17,7 @@
 #include "../setup.h"
 #include "../worktree.h"
 #include "../wrapper.h"
+#include "../write-or-die.h"
 
 /*
  * This backend uses the following flags in `ref_update::flags` for