system/qtest: Include missing 'hw/core/cpu.h' header

"hw/core/cpu.h" declares 'first_cpu'. Include it to avoid
when unrelated headers are refactored:

  system/qtest.c:548:33: error: use of undeclared identifier 'first_cpu'
            address_space_write(first_cpu->as, addr, MEMTXATTRS_UNSPECIFIED,
                                ^

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20231212113016.29808-2-philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

system/qtest.c

@@ -21,6 +21,7 @@
 #include "exec/tswap.h"
 #include "hw/qdev-core.h"
 #include "hw/irq.h"
+#include "hw/core/cpu.h"
 #include "qemu/accel.h"
 #include "sysemu/cpu-timers.h"
 #include "qemu/config-file.h"