Fixing memory leak introduced in previous commit.

Commit introduced the leak: "Removing XMA legacy from VP9 code."
I5e9dfc50b6312848df8c3b8276f6b4d0c7ff9cf9

Change-Id: Ia1f91bb8403bef340de9f089fa78b3fccc3e9917

vp9/vp9_dx_iface.c

@@ -89,6 +89,8 @@ static vpx_codec_err_t decoder_destroy(vpx_codec_alg_priv_t *ctx) {
     ctx->pbi = NULL;
   }
 
+  vpx_free(ctx);
+
   return VPX_CODEC_OK;
 }