stkutil: Add Play Tone response builder support

src/stkutil.c

@@ -3209,6 +3209,7 @@ unsigned int stk_pdu_from_response(const struct stk_response *response,
 					NULL);
 		break;
 	case STK_COMMAND_TYPE_SEND_SMS:
+	case STK_COMMAND_TYPE_PLAY_TONE:
 		break;
 	default:
 		return 0;

src/stkutil.h

@@ -1026,6 +1026,7 @@ struct stk_response {
 		struct stk_response_generic display_text;
 		struct stk_response_get_inkey get_inkey;
 		struct stk_response_get_input get_input;
+		struct stk_response_generic play_tone;
 		struct stk_response_generic send_sms;
 	};