meson: Use osdep_prefix for strchrnul()

meson: Use osdep_prefix for strchrnul()

macOS SDK may have the symbol of strchrnul(), but it is actually
available only on macOS 15.4 or later and that fact is codified in
string.h. Include the header file using osdep_prefix to check if the
function is available on the deployment target.

Cc: qemu-stable@nongnu.org
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20250424-buildsys-v1-4-97655e3b25d7@daynix.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>