pkt-line: annotate packet_writel with LAST_ARG_MUST_BE_NULL

pkt-line: annotate packet_writel with LAST_ARG_MUST_BE_NULL

packet_writel() takes a variable-sized list and reads to
the first NULL. Let's let the compiler know so that it can
help us catch mistakes in the callers.

This should have been annotated similarly when it was a
static function, but it's doubly important now that the
function is available to the whole code-base.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>