core: Make ostree_set_xattrs() private

Nothing external uses it.  We keep ostree_get_xattrs_for_file() public
because it's convenient for external consumers to get xattrs in
exactly the format we desire.

https://bugzilla.gnome.org/show_bug.cgi?id=707733

src/libostree/ostree-core.c

@@ -1073,8 +1073,8 @@ _ostree_set_xattrs_fd (int            fd,
   return ret;
 }
 
-/**
- * ostree_set_xattrs:
+/*
+ * _ostree_set_xattrs:
  * @f: a file
  * @xattrs: Extended attribute list
  * @cancellable: Cancellable
@@ -1085,10 +1085,10 @@ _ostree_set_xattrs_fd (int            fd,
  * attributes.
  */
 gboolean
-ostree_set_xattrs (GFile  *f, 
-                   GVariant *xattrs, 
-                   GCancellable *cancellable, 
-                   GError **error)
+_ostree_set_xattrs (GFile  *f, 
+                    GVariant *xattrs, 
+                    GCancellable *cancellable, 
+                    GError **error)
 {
   const char *path;
   gboolean ret = FALSE;