Make audio work by default on OpenBSD.

Index: src/core/control/settings/Settings.h
--- src/core/control/settings/Settings.h.orig
+++ src/core/control/settings/Settings.h
@@ -393,11 +393,11 @@ class Settings { (public)
     fs::path const& getAudioFolder() const;
     void setAudioFolder(fs::path audioFolder);
 
-    static constexpr PaDeviceIndex AUDIO_INPUT_SYSTEM_DEFAULT = -1;
+    static constexpr PaDeviceIndex AUDIO_INPUT_SYSTEM_DEFAULT = 0;
     PaDeviceIndex getAudioInputDevice() const;
     void setAudioInputDevice(PaDeviceIndex deviceIndex);
 
-    static constexpr PaDeviceIndex AUDIO_OUTPUT_SYSTEM_DEFAULT = -1;
+    static constexpr PaDeviceIndex AUDIO_OUTPUT_SYSTEM_DEFAULT = 0;
     PaDeviceIndex getAudioOutputDevice() const;
     void setAudioOutputDevice(PaDeviceIndex deviceIndex);
 
