stream_encoder.c: Make compression_levels_ array const

As pointed out by github user @randomascii, the `compression_levels_`
struct could and should be const.

Closes: https://github.com/xiph/flac/issues/26

src/libFLAC/stream_encoder.c

@@ -99,7 +99,7 @@ typedef enum {
 	ENCODER_IN_AUDIO = 2
 } EncoderStateHint;
 
-static struct CompressionLevels {
+static const  struct CompressionLevels {
 	FLAC__bool do_mid_side_stereo;
 	FLAC__bool loose_mid_side_stereo;
 	unsigned max_lpc_order;