pktgen: Remove fill_imix_distribution() CONFIG_XFRM dependency

Currently, the declaration of fill_imix_distribution() is dependent
on CONFIG_XFRM. This is incorrect.

Move fill_imix_distribution() declaration out of #ifndef CONFIG_XFRM
block.

Signed-off-by: Nick Richardson <richardsonnick@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>