meson: Add -Werror=gnu-empty-initializer to MSVC compat args

meson: Add -Werror=gnu-empty-initializer to MSVC compat args

Only clang has this argument (at least as of clang 8 and gcc 9), which
errors when using the gcc empty initializer syntax in C:

```C
struct foo f = {};
```

GCC has a warning for this, but only when using -Wpedantic, which is a
lot of noise to lose useful warnings in.

Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>