Rename parameter if_index to stop clang warning:

capture_opts.c:1017:61: error: declaration of 'index' shadows a global declaration [-Werror=shadow]

Change-Id: Ie409b4fa7abeb85e460bea398735cdc98d9034b1
Reviewed-on: https://code.wireshark.org/review/3041
Reviewed-by: Graham Bloice <graham.bloice@trihedral.com>