core: Make OSTREE_TIMESTAMP public API

This way e.g. flatpak can detect which timestamp it should use.
See `flatpak/common/flatpak-utils.c:flatpak_zero_mtime`.

Closes: #501
Approved by: cgwalters

src/libostree/ostree-repo-private.h

@@ -32,8 +32,6 @@ G_BEGIN_DECLS
 #define _OSTREE_SUMMARY_CACHE_DIR "summaries"
 #define _OSTREE_CACHE_DIR "cache"
 
-#define OSTREE_TIMESTAMP (0)
-
 typedef enum {
   OSTREE_REPO_TEST_ERROR_PRE_COMMIT = (1 << 0)
 } OstreeRepoTestErrorFlags;