libavformat: Fix a faulty api deprecation guard in prepare_input_packet

This seems to have been added by mistake in 11de006b, by not
noticing the negation for the existing condition. This block does
not contain any code that accesses the codec field in AVStream.

This function is meant to serve as a complement to compute_pkt_fields2,
which is guarded by FF_API_COMPUTE_PKT_FIELDS2 && FF_API_LAVF_AVCTX.

Signed-off-by: Martin Storsjö <martin@martin.st>
