scanner: Reword fallthrough comment to quiet GCC

GCC 7 now requires an explicit comment noting that case statements
without a break fall through. We already had one of those in the
scanner, but GCC wasn't smart enough to pick it up.

Quiet the warning by making the comment less elaborate.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>
diff --git a/src/scanner.c b/src/scanner.c
index a6c334f..517068c 100644
--- a/src/scanner.c
+++ b/src/scanner.c
@@ -740,9 +740,7 @@
 		switch (arg->type) {
 		case NEW_ID:
 			ctx->message->new_id_count++;
-
-			/* Fall through to OBJECT case. */
-
+			/* fallthrough */
 		case OBJECT:
 			if (interface_name)
 				arg->interface_name = xstrdup(interface_name);