Fixed warning C4210: nonstandard extension used: function given file scope

Resurrected SDL_audio_c.h, we knew it would be back...

src/audio/SDL_audio_c.h

@@ -0,0 +1,27 @@
+/*
+  Simple DirectMedia Layer
+  Copyright (C) 1997-2023 Sam Lantinga <slouken@libsdl.org>
+
+  This software is provided 'as-is', without any express or implied
+  warranty.  In no event will the authors be held liable for any damages
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+*/
+
+#ifndef SDL_audio_c_h_
+#define SDL_audio_c_h_
+
+extern void SDL_UpdateAudio(void);
+
+#endif /* SDL_audio_c_h_ */

src/events/SDL_events.c

@@ -24,6 +24,7 @@
 
 #include "SDL_events_c.h"
 #include "../SDL_hints_c.h"
+#include "../audio/SDL_audio_c.h"
 #include "../timer/SDL_timer_c.h"
 #ifndef SDL_JOYSTICK_DISABLED
 #include "../joystick/SDL_joystick_c.h"
@@ -862,7 +863,6 @@ static void SDL_PumpEventsInternal(SDL_bool push_sentinel)
     }
 
 #ifndef SDL_AUDIO_DISABLED
-    extern void SDL_UpdateAudio(void);  // this is internal-only, so it doesn't have a hint and is not a public API.
     SDL_UpdateAudio();
 #endif