repo: Fix object storage size API to be 64 bit

This fixes a pointer size warning on 32-bit builds.

https://bugzilla.gnome.org/show_bug.cgi?id=706235

src/libostree/ostree-repo.c

@@ -2361,7 +2361,7 @@ gboolean
 ostree_repo_query_object_storage_size (OstreeRepo           *self,
                                        OstreeObjectType      objtype,
                                        const char           *sha256, 
-                                       gsize                *out_size,
+                                       guint64              *out_size,
                                        GCancellable         *cancellable,
                                        GError              **error)
 {

src/libostree/ostree-repo.h

@@ -216,7 +216,7 @@ gboolean ostree_repo_load_object_stream (OstreeRepo         *self,
 gboolean      ostree_repo_query_object_storage_size (OstreeRepo           *self,
                                                      OstreeObjectType      objtype,
                                                      const char           *sha256, 
-                                                     gsize                *out_size,
+                                                     guint64              *out_size,
                                                      GCancellable         *cancellable,
                                                      GError              **error);