Fix build for --disable-encode

Seems to only effect OS X according to Travis CI

lib/encoder_disabled.c

@@ -17,6 +17,9 @@
 #include "apiwrapper.h"
 #include "encint.h"
 
+const th_quant_info TH_VP31_QUANT_INFO = {};
+const th_huff_code TH_VP31_HUFF_CODES[TH_NHUFFMAN_TABLES][TH_NDCT_TOKENS];
+
 th_enc_ctx *th_encode_alloc(const th_info *_info){
   return NULL;
 }