avconv: set the output stream timebase

This is required by the new API.

avconv.c

@@ -1972,6 +1972,8 @@ static int transcode_init(void)
                    "Error initializing the output stream codec context.\n");
             exit_program(1);
         }
+
+        ost->st->time_base = ost->enc_ctx->time_base;
     }
 
     /* init input streams */