osdep/hurd/getroot: Fix 64-bit build

The file_get_fs_options() takes a mach_msg_type_number_t, 32-bit,
not a size_t, 64-bit on 64-bit platforms.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Reviewed-by: Vladimir Serbinenko <phcoder@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/osdep/hurd/getroot.c

@@ -58,7 +58,7 @@ grub_util_find_hurd_root_device (const char *path)
   file_t file;
   error_t err;
   char *argz = NULL, *name = NULL, *ret;
-  size_t argz_len = 0;
+  mach_msg_type_number_t argz_len = 0;
   int i;
 
   file = file_name_lookup (path, 0, 0);