(CTR) Buildfix

frontend/drivers/platform_ctr.c

@@ -396,6 +396,8 @@ void gfxSetFramebufferInfo(gfxScreen_t screen, u8 id);
 static void frontend_ctr_init(void* data)
 {
 #ifndef IS_SALAMANDER
+   extern audio_driver_t audio_null;
+
    (void)data;
 
    verbosity_enable();

retroarch.c

@@ -254,7 +254,7 @@
       | DRIVER_LED_MASK \
       | DRIVER_MIDI_MASK )
 
-static audio_driver_t audio_null = {
+audio_driver_t audio_null = {
    NULL, /* init */
    NULL, /* write */
    NULL, /* stop */