Wrap palette code inside CONFIG_PALETTE flag.

This flag was already added to aomedia/master, so bringing it back to
webm/nextgenv2, as part of an effort to get the two codebases in sync.

Change-Id: I2b933a6a160e4210d1411a9e7978149eb8553205
diff --git a/av1/decoder/detokenize.c b/av1/decoder/detokenize.c
index 9a40f69..22d90db 100644
--- a/av1/decoder/detokenize.c
+++ b/av1/decoder/detokenize.c
@@ -332,6 +332,7 @@
   }
 }
 
+#if CONFIG_PALETTE
 void av1_decode_palette_tokens(MACROBLOCKD *const xd, int plane,
                                aom_reader *r) {
   MODE_INFO *const mi = xd->mi[0];
@@ -360,6 +361,7 @@
     }
   }
 }
+#endif  // CONFIG_PALETTE
 
 int av1_decode_block_tokens(MACROBLOCKD *const xd, int plane,
                             const scan_order *sc, int x, int y, TX_SIZE tx_size,