target/xtensa: Include missing 'qemu/atomic.h' header

Since commit fa92bd4a ("target/xtensa: fix access to
the INTERRUPT SR") these files use QEMU atomic API.
Explicit the header inclusion instead of relying on
implicit and indirect inclusion.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230828221314.18435-10-philmd@linaro.org>