s390x/kvm: include hw_accel.h instead of kvm.h

s390x/kvm: include hw_accel.h instead of kvm.h

Commit b394662 ("kvm: move cpu synchronization code") switched
to hw_accel.h instead of kvm.h, but missed s390x, resulting in

  CC      s390x-softmmu/target/s390x/kvm.o
/home/cohuck/git/qemu/target/s390x/kvm.c: In function ‘kvm_sclp_service_call’:
/home/cohuck/git/qemu/target/s390x/kvm.c:1034:5: error: implicit declaration of function ‘cpu_synchronize_state’ [-Werror=implicit-function-declaration]
     cpu_synchronize_state(CPU(cpu));
     ^
/home/cohuck/git/qemu/target/s390x/kvm.c:1034:5: error: nested extern declaration of ‘cpu_synchronize_state’ [-Werror=nested-externs]
/home/cohuck/git/qemu/target/s390x/kvm.c: In function ‘sigp_initial_cpu_reset’:
/home/cohuck/git/qemu/target/s390x/kvm.c:1628:5: error: implicit declaration of function ‘cpu_synchronize_post_reset’ [-Werror=implicit-function-declaration]
     cpu_synchronize_post_reset(cs);
     ^
/home/cohuck/git/qemu/target/s390x/kvm.c:1628:5: error: nested extern declaration of ‘cpu_synchronize_post_reset’ [-Werror=nested-externs]
/home/cohuck/git/qemu/target/s390x/kvm.c: In function ‘sigp_set_prefix’:
/home/cohuck/git/qemu/target/s390x/kvm.c:1665:5: error: implicit declaration of function ‘cpu_synchronize_post_init’ [-Werror=implicit-function-declaration]
     cpu_synchronize_post_init(cs);
     ^
/home/cohuck/git/qemu/target/s390x/kvm.c:1665:5: error: nested extern declaration of ‘cpu_synchronize_post_init’ [-Werror=nested-externs]
cc1: all warnings being treated as errors
/home/cohuck/git/qemu/rules.mak:64: recipe for target 'target/s390x/kvm.o' failed

Fix this.

Fixes: b394662 ("kvm: move cpu synchronization code")
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Acked-by: Vincent Palatin <vpalatin@chromium.org>