audio_format: allow 32 bit samples

This is the first patch in a series to enable 32 bit audio samples in
MPD.  32 bit samples are more tricky than 24 bit samples, because the
integer may overflow when you operate on a sample.

src/audio_format.h

@@ -59,7 +59,7 @@ audio_valid_sample_rate(unsigned sample_rate)
 static inline bool
 audio_valid_sample_format(unsigned bits)
 {
-	return bits == 16 || bits == 24 || bits == 8;
+	return bits == 16 || bits == 24 || bits == 32 || bits == 8;
 }
 
 /**