dd: do not have 'ocount' variable if ibs/obs support is not enabled

dd: do not have 'ocount' variable if ibs/obs support is not enabled

function                                             old     new   delta
packed_usage                                       32964   32961      -3
dd_main                                             1033    1021     -12
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(add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-15)             Total: -15 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>