aviocat: Flush the output before closing

Without this, the end of the file might not be transmitted/written.
Signed-off-by: Martin Storsjö <martin@martin.st>

tools/aviocat.c

@@ -89,6 +89,7 @@ int main(int argc, char **argv)
         }
     }
 
+    avio_flush(output);
     avio_close(output);
 fail:
     avio_close(input);