move SoundIoBackendDummy enum item position

Now new backends can go at the end position and avoid rolling
major version number. When dummy was the last one, it was tempting
to put new backends before it.

soundio/soundio.h

@@ -217,12 +217,12 @@ enum SoundIoChannelLayoutId {
 
 enum SoundIoBackend {
     SoundIoBackendNone,
+    SoundIoBackendDummy,
     SoundIoBackendJack,
     SoundIoBackendPulseAudio,
     SoundIoBackendAlsa,
     SoundIoBackendCoreAudio,
     SoundIoBackendWasapi,
-    SoundIoBackendDummy,
 };
 
 enum SoundIoDeviceAim {