meson: Don't set hidden visibility on windows

Normally, hidden visibility has no effect on Windows. Therefore, there's usually no point in trying to enable it, or warning about all symbols being public.

Additionally, the CHECKASM_API define in attributes.h, which is meant to counter the hidden visibility, doesn't ever try to set the default visibility on Windows; on that platform it only chooses between dllexport or nothing.

However - with Clang in mingw mode, setting hidden visibility actually does have an effect - explicitly excluding symbols from the automatic export of all symbols if no dllexport attributes are present. So if we do use dllexports, setting the hidden visibility makes no difference, but if we don't use dllexports (we currently don't yet enable that properly), the hidden visibility breaks exporting symbols.

Therefore, in short, this change fixes building DLLs with mingw tools with Clang. (As long as the checkasm selftest includes all the libcheckasm objects directly, this isn't noticed though.)