extensions: DNAT: Rename from libipt to libxt

Prepare for merge of libipt and libip6t DNAT extensions, allow for
better code review.

Signed-off-by: Phil Sutter <phil@nwl.cc>

diff --git a/extensions/libipt_DNAT.c b/extensions/libxt_DNAT.c
similarity index 100%
rename from extensions/libipt_DNAT.c
rename to extensions/libxt_DNAT.c