Require an --experimental-bitstream flag at runtime for encoding profile 1.

Change-Id: I73865bbbf6ea74434f15b73783ae97b4d273be80
diff --git a/vpxenc.h b/vpxenc.h
index 1e6acaa..a8c3722 100644
--- a/vpxenc.h
+++ b/vpxenc.h
@@ -46,6 +46,7 @@
   int show_rate_hist_buckets;
   int disable_warnings;
   int disable_warning_prompt;
+  int experimental_bitstream;
 };
 
 #ifdef __cplusplus