The C# flag table added by commit 9b06c22648 (VS: Add flag tables for
C#, 2016-12-01, v3.8.0-rc1~230^2) and preserved by commit 6e947179a8
(Add json flag tables for C#, 2018-11-21, v3.14.0-rc1~282^2~27) was
written by hand. Fix the entry name for the `NoStdLib` property.
Fixes: #24077