audio: use native OpenAL Soft typedefs, drop local token fallbacks

Now that ref/openal ships a current alext.h, include it and use the real
LPALC*SOFT / ALCEVENTPROCTYPESOFT typedefs instead of hand-rolled function
pointer types and #define'd ALC_* tokens. The event callback is marked
ALC_APIENTRY + noexcept to match ALCEVENTPROCTYPESOFT. No behaviour change.

audio/audio.h

@@ -15,6 +15,7 @@ http://mozilla.org/MPL/2.0/.
 #else
 #include <AL/al.h>
 #include <AL/alc.h>
+#include <AL/alext.h>
 #endif
 
 namespace audio {