e1_input: change prototype of ->sign_link(...)

This patch removes the struct e1inp_sign_link parameter since this
is already available in the msgb->dst field of the message.
diff --git a/src/input/hsl.c b/src/input/hsl.c
index aa79cdf..d8eb8db 100644
--- a/src/input/hsl.c
+++ b/src/input/hsl.c
@@ -186,7 +186,7 @@
 			"no action set for signalling messages.\n");
 		return -ENOENT;
 	}
-	e1i_ts->line->ops->sign_link(msg, link);
+	e1i_ts->line->ops->sign_link(msg);
 
 	return ret;
 }