Fix compilation warning due to incorrectly specified type

In audio/coreaudio.c, a variable named "str" was assigned "const char" values,
which resulted in the following warnings:

-----8<-----
audio/coreaudio.c: In function ‘coreaudio_logstatus’:
audio/coreaudio.c:59: warning: initialization discards qualifiers from pointer target type
audio/coreaudio.c:63: warning: assignment discards qualifiers from pointer target type
(...)
-----8<-----
Signed-off-by: NAlexandre Raymond <cerbere@gmail.com>
Acked-by: NStefan Weil <weil@mail.berlios.de>
Signed-off-by: NAndreas Färber <andreas.faerber@web.de>

audio/coreaudio.c

@@ -56,7 +56,7 @@ typedef struct coreaudioVoiceOut {
 
 static void coreaudio_logstatus (OSStatus status)
 {
-    char *str = "BUG";
+    const char *str = "BUG";
 
     switch(status) {
     case kAudioHardwareNoError: