sndio: Add module.

- can control sndio: change volume, toggle mute
- appearance is somewhat dynamic: takes muted status into account
- uses polling inside sleeper thread to update values
- uses sioctl_* functions, requires sndio>=1.7.0.

src/factory.cpp

@@ -76,6 +76,11 @@ waybar::AModule* waybar::Factory::makeModule(const std::string& name) const {
     if (ref == "mpd") {
       return new waybar::modules::MPD(id, config_[name]);
     }
+#endif
+#ifdef HAVE_LIBSNDIO
+    if (ref == "sndio") {
+      return new waybar::modules::Sndio(id, config_[name]);
+    }
 #endif
     if (ref == "temperature") {
       return new waybar::modules::Temperature(id, config_[name]);