ao_oss: define PATH_DEV_MIXER as it is an internal define

This fixes a mismatch between configure working and build time
failing with Linux + OSSv4, enabling compilation on Debian based
Linux systems with the oss4-dev package.

Fixes #9378

audio/out/ao_oss.c

@@ -49,6 +49,7 @@
 #endif
 
 #define PATH_DEV_DSP "/dev/dsp"
+#define PATH_DEV_MIXER "/dev/mixer"
 
 struct priv {
     int dsp_fd;