convert: mark unused parameter in null stream filter

The null stream filter unsurprisingly does not look at its "filter"
argument, since it just eats bytes. But we can't drop it, since it has
to conform to the same virtual interface that real filters do. Mark the
unused parameter to appease -Wunused-parameter.

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

convert.c

@@ -1549,7 +1549,7 @@ struct stream_filter {
 	struct stream_filter_vtbl *vtbl;
 };
 
-static int null_filter_fn(struct stream_filter *filter,
+static int null_filter_fn(struct stream_filter *filter UNUSED,
 			  const char *input, size_t *isize_p,
 			  char *output, size_t *osize_p)
 {
@@ -1568,7 +1568,7 @@ static int null_filter_fn(struct stream_filter *filter,
 	return 0;
 }
 
-static void null_free_fn(struct stream_filter *filter)
+static void null_free_fn(struct stream_filter *filter UNUSED)
 {
 	; /* nothing -- null instances are shared */
 }