qemu/coroutine: Include missing 'qemu/atomic.h' header

qemu/coroutine: Include missing 'qemu/atomic.h' header

Commit 944f3d5dd21 ("coroutine: Add qemu_co_mutex_assert_locked")
added an inline method which uses qatomic_read(), itself declared
in "qemu/atomic.h". Explicitly include it now to avoid issue when
refactoring unrelated headers.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Message-Id: <20241217141326.98947-3-philmd@linaro.org>