jsonrpc: don't be coy with details for command_its_complicated().

Obviously the Facebook relationship status joke was a bit subtle, but I've
continued it anyway because I'm especially susceptible to Dad jokes.

Suggested-by: @niftynei
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/jsonrpc.c

@@ -57,7 +57,8 @@ struct command_result *command_param_failed(void)
 	return &param_failed;
 }
 
-struct command_result *command_its_complicated(void)
+struct command_result *command_its_complicated(const char *relationship_details
+					       UNNEEDED)
 {
 	return &unknown;
 }