As the default option for msvc 2019 does support designated initializers
```
../src/util/tests/timespec_test.cpp(302): error C7555: use of designated initializers requires at least '/std:c++20'
../src/util/tests/timespec_test.cpp(303): error C7555: use of designated initializers requires at least '/std:c++20'
../src/util/tests/timespec_test.cpp(312): error C7555: use of designated initializers requires at least '/std:c++20'
../src/util/tests/timespec_test.cpp(313): error C7555: use of designated initializers requires at least '/std:c++20'
```
Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15497>
