Add clock function to call interface

Implementations will have a clock to play tones, if no clock is
available from a transceiver.

src/bnetz/bnetz.c

@@ -862,5 +862,7 @@ void call_down_audio(int callref, sample_t *samples, int count)
 	}
 }
 
+void call_down_clock(void) {}
+
 void dump_info(void) {}