meson: disable some more warnings on msvc

These warnings triggers for me, and they are harmless as-is. Let's
disable them to avoid hiding actually scary warnings.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Charmaine Lee <charmainel@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4343>

meson.build

@@ -933,7 +933,9 @@ if cc.get_id() == 'msvc'
                '/wd4756',  # overflow in constant arithmetic
                '/wd4800',  # forcing value to bool 'true' or 'false' (performance warning)
                '/wd4996',  # disabled deprecated POSIX name warnings
-               '/wd4291']  # no matching operator delete found
+               '/wd4291',  # no matching operator delete found
+               '/wd4146',  # unary minus operator applied to unsigned type, result still unsigned
+               '/wd4200']  # nonstandard extension used: zero-sized array in struct/union
     if cc.has_argument(a)
       c_args += a
     endif