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/encoder/block.h b/av1/encoder/block.h
index 8a6e134..3b3f93a 100644
--- a/av1/encoder/block.h
+++ b/av1/encoder/block.h
@@ -66,10 +66,12 @@
 #endif
 } MB_MODE_INFO_EXT;
 
+#if CONFIG_PALETTE
 typedef struct {
   uint8_t best_palette_color_map[MAX_SB_SQUARE];
   float kmeans_data_buf[2 * MAX_SB_SQUARE];
 } PALETTE_BUFFER;
+#endif  // CONFIG_PALETTE
 
 typedef struct macroblock MACROBLOCK;
 struct macroblock {
@@ -128,7 +130,9 @@
   int *nmvsadcost_hp[2];
   int **mvsadcost;
 
+#if CONFIG_PALETTE
   PALETTE_BUFFER *palette_buffer;
+#endif  // CONFIG_PALETTE
 
   // These define limits to motion vector components to prevent them
   // from extending outside the UMV borders