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>

Author: peff

pkt-line.h

@@ -25,6 +25,7 @@ void packet_buf_flush(struct strbuf *buf);
 void packet_buf_write(struct strbuf *buf, const char *fmt, ...) __attribute__((format (printf, 2, 3)));
 int packet_flush_gently(int fd);
 int packet_write_fmt_gently(int fd, const char *fmt, ...) __attribute__((format (printf, 2, 3)));
+LAST_ARG_MUST_BE_NULL
 int packet_writel(int fd, const char *line, ...);
 int write_packetized_from_fd(int fd_in, int fd_out);
 int write_packetized_from_buf(const char *src_in, size_t len, int fd_out);