tcg/tci: Disable -Wundef FFI_GO_CLOSURES warning

tcg/tci: Disable -Wundef FFI_GO_CLOSURES warning

Since we build TCI with FFI (commit 22f15579fa1 "tcg: Build ffi data
structures for helpers") we get on Darwin:

  In file included from ../../tcg/tci.c:22:
  In file included from include/tcg/helper-info.h:13:
  /Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/ffi/ffi.h:483:5: warning: 'FFI_GO_CLOSURES' is not defined, evaluates to 0 [-Wundef]
    483 | #if FFI_GO_CLOSURES
        |     ^
  1 warning generated.

This was fixed in upstream libffi in 2023, but not backported to MacOSX.
Simply disable the warning locally.

Reported-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>