[gdb/dap] Flush after printing in log_stack

I noticed that function log flushes the dap log file after printing, but
that function log_stack doesn't.

Fix this by also flushing the dap log file in log_stack.

Tested on aarch64-linux.

Approved-By: Tom Tromey <tom@tromey.com>

gdb/python/lib/gdb/dap/startup.py

@@ -178,6 +178,7 @@ def log_stack(level=LogLevel.DEFAULT):
     """Log a stack trace to the log file, if logging is enabled."""
     if dap_log.log_file is not None and level <= _log_level.value:
         traceback.print_exc(file=dap_log.log_file)
+        dap_log.log_file.flush()
 
 
 @in_gdb_thread