parse-options: add cast to correct pointer type to OPT_SET_PTR

Do not force users of OPT_SET_PTR to cast pointer to correct
underlying pointer type by integrating cast into OPT_SET_PTR macro.

Cast is required to prevent 'initialization makes integer from pointer
without a cast' compiler warning.

Signed-off-by: Junio C Hamano <gitster@pobox.com>