[Bug-tar] warning when compiled with --disable-nls

[Gilles Espinasse]

when tar-1.23 is compiled with --disable-nls using gcc-4.4.3, this warning
is send.
  CC     warning.o
tar.c: In function 'tar_help_filter':
tar.c:1290: warning: assignment discards qualifiers from pointer target type
tar.c:1294: warning: assignment discards qualifiers from pointer target type
  CCLD   tar

That's a bit out of my small C knowledge to fix that.

Gilles