dl: Fix grub_dl_is_persistent() for emu

dl: Fix grub_dl_is_persistent() for emu

When attempting to build grub-emu the compilation failed with the
following error message:

  include/grub/dl.h: In function ‘grub_dl_is_persistent’:
  include/grub/dl.h:262:1: error: no return statement in function returning non-void [-Werror=return-type]

To avoid the error make the function always return 0.

Fixes: ba8eadde6be1 (dl: Provide a fake grub_dl_set_persistent() and grub_dl_is_persistent() for the emu target)
Signed-off-by: Gary Lin <glin@suse.com>
Cc: Daniel Axtens <dja@axtens.net>
Cc: Sudhakar Kuppusamy <sudhakar@linux.ibm.com>
Reviewed-by: Sudhakar Kuppusamy <sudhakar@linux.ibm.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>