aout: fix f7881537

Make char explicitely signed to represent a boolean
If it is unsigned, default value -1 will represent 255 and
effectively set mute to 255, i.e. true.

Fix lack of hearable audio output on android

src/audio_output/aout_internal.h

@@ -57,7 +57,7 @@ typedef struct
         vlc_mutex_t lock;
         char *device;
         float volume;
-        char mute;
+        signed char mute;
     } req;
 
     struct