build: Only define OS_OBJECT_USE_OBJC with gcc

Recent versions of macOS use clang instead of gcc. The OS_OBJECT_USE_OBJC
define is only necessary when building with gcc. Let's not define it when
building with clang.

With this patch, I can successfully include GCD headers in QEMU when
building with clang.

Signed-off-by: Alexander Graf <graf@amazon.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Message-ID: <20230830161425.91946-2-graf@amazon.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>