Fix audio system index to string for Portaudio.

diff --git a/src/lingot-config.c b/src/lingot-config.c
index 48213da..8d57596 100644
--- a/src/lingot-config.c
+++ b/src/lingot-config.c
@@ -50,7 +50,7 @@ const char* option_formats = "mssddffdffffddfddf|d";

 // converts an audio_system_t to a string
 const char* audio_system_t_to_str(audio_system_t audio_system) {
-	const char* values[] = { "OSS", "ALSA", "JACK" };
+	const char* values[] = { "OSS", "ALSA", "JACK", "PORTAUDIO" };
 	return values[audio_system];
 }