static_assert is only usable from C++

[ccawley2011]

OpenWatcom currently supports static_assert when building with C++11, but is currently unavailable in C code. In C23, static_assert is a keyword like in C++11, while in C11 it's a macro defined in assert.h that maps onto the _Static_assert keyword.

https://en.cppreference.com/w/c/language/static_assert.html