vpxdec: quiet signed/unsigned warning

Change-Id: I93c56dfa547af9b2f2b96c4f85fd9862ea67af62

vpxdec.c

diff --git a/vpxdec.c b/vpxdec.c
index fde3b9a..285d58e 100644
--- a/vpxdec.c
+++ b/vpxdec.c
@@ -562,7 +562,7 @@
   int                     opt_i420 = 0;
   vpx_codec_dec_cfg_t     cfg = {0, 0, 0};
 #if CONFIG_VP9_HIGHBITDEPTH
-  int                     output_bit_depth = 0;
+  unsigned int            output_bit_depth = 0;
 #endif
 #if CONFIG_VP8_DECODER
   vp8_postproc_cfg_t      vp8_pp_cfg = {0};