compile warning with Visual Studio 2022

[davidpanderson]

1>C:\Users\davea\Documents\BOINC_git\boinc\samples\docker_wrapper\toml.h(237,24): warning C4996: 'gmtime': This function or variable may be unsafe. Consider using gmtime_s instead. To disable deprecation, use _CRT_SECURE_NO_WARNINGS. See online help for details.