Set default sound backend on OpenBSD to cubeb

Set the default sound backend to cubeb instead of the default
being none.

Source/Core/AudioCommon/AudioCommon.cpp

@@ -100,7 +100,7 @@ std::string GetDefaultSoundBackend()
 #elif defined __linux__
   if (AlsaSound::IsValid())
     backend = BACKEND_ALSA;
-#elif defined(__APPLE__) || defined(_WIN32)
+#elif defined(__APPLE__) || defined(_WIN32) || defined(__OpenBSD__)
   backend = BACKEND_CUBEB;
 #endif
   return backend;