Add clock function to call interface

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

src/test/test_dms.c

@@ -253,5 +253,7 @@ int main(void)
 	return 0;
 }
 
+void call_down_clock(void) {}
+
 void print_image(void) {}
 

src/test/test_sms.c

@@ -162,5 +162,7 @@ int main(void)
 	return 0;
 }
 
+void call_down_clock(void) {}
+
 void print_image(void) {}