Fix compilation warning due to incorrectly specified type

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: Alexandre Raymond <[email protected]>
Acked-by: Stefan Weil <[email protected]>
Signed-off-by: Andreas Färber <[email protected]>