log: fix memory leak when log to file is enabled

When log to file is enabled, the file is not closed by OpenOCD at
exit. This is reported by Valgrind as a memory leak that is still
reachable, as the internal buffers of 'FILE *log_output' are freed
by the automatic fclose() at exit.

Close the log file before exit.

Change-Id: Id472c0d04462035254a9b49ecb0a4037263c6f6f
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6789
Tested-by: jenkins

src/helper/log.h

@@ -72,6 +72,7 @@ __attribute__ ((format (PRINTF_ATTRIBUTE_FORMAT, 5, 6)));
  * Initialize logging module.  Call during program startup.
  */
 void log_init(void);
+void log_exit(void);
 int set_log_output(struct command_context *cmd_ctx, FILE *output);
 
 int log_register_commands(struct command_context *cmd_ctx);