make ostree_cmd__private__ ret value const

src/libostree/ostree-cmdprivate.c

@@ -40,7 +40,7 @@ impl_ostree_generate_grub2_config (OstreeSysroot *sysroot, int bootversion, int
  * Do not call this function; it is used to share private API between
  * the OSTree commandline and the library.
  */
-OstreeCmdPrivateVTable *
+const OstreeCmdPrivateVTable *
 ostree_cmd__private__ (void)
 {
   static OstreeCmdPrivateVTable table = {

src/libostree/ostree-cmdprivate.h

@@ -28,7 +28,7 @@ typedef struct {
   gboolean (* ostree_generate_grub2_config) (OstreeSysroot *sysroot, int bootversion, int target_fd, GCancellable *cancellable, GError **error);
 } OstreeCmdPrivateVTable;
 
-OstreeCmdPrivateVTable *
+const OstreeCmdPrivateVTable *
 ostree_cmd__private__ (void);
 
 G_END_DECLS