Fix compilation if VBOX_WITH_AUDIO_OSS is defined while VBOX_WITH_AUDIO_PULSE and VBOX_WITH_AUDIO_ALSA are not

--- a/src/VBox/Main/xml/Settings.cpp
+++ b/src/VBox/Main/xml/Settings.cpp
@@ -8931,6 +8931,9 @@
     RTCLock lock(s_mtx);
     if (s_enmLinuxDriver == AudioDriverType_Null)
     {
+# ifdef VBOX_WITH_AUDIO_OSS
+	s_enmLinuxDriver = AudioDriverType_OSS;
+# endif /* VBOX_WITH_AUDIO_OSS */
 # ifdef VBOX_WITH_AUDIO_PULSE
         /* Check for the pulse library & that the pulse audio daemon is running. */
         if (RTProcIsRunningByName("pulseaudio") &&
@@ -8943,10 +8946,7 @@
              if (RTLdrIsLoadable("libasound.so.2"))
                 s_enmLinuxDriver = AudioDriverType_ALSA;
 # endif /* VBOX_WITH_AUDIO_ALSA */
-# ifdef VBOX_WITH_AUDIO_OSS
-             else
-                s_enmLinuxDriver = AudioDriverType_OSS;
-# endif /* VBOX_WITH_AUDIO_OSS */
+	;
     }
     return s_enmLinuxDriver;