DL window: constify resend_needed() function

It doesn't change any state so mark it as const.

Change-Id: I5d672bfd654198aebb187772de464c52b3209435
diff --git a/src/rlc.h b/src/rlc.h
index ee86c2a..14d2082 100644
--- a/src/rlc.h
+++ b/src/rlc.h
@@ -303,7 +303,7 @@
 	const uint16_t distance() const;
 
 	/* Methods to manage reception */
-	int resend_needed();
+	int resend_needed() const;
 	int mark_for_resend();
 	void update(BTS *bts, char *show_rbb, uint16_t ssn,
 			uint16_t *lost, uint16_t *received);