Fixes gcc '-Wunused-but-set-variable' warning:

arch/x86/kvm/x86.c: In function kvm_make_scan_ioapic_request_mask:
arch/x86/kvm/x86.c:7911:7: warning: variable called set but not
used [-Wunused-but-set-variable]

It is not used since commit 7ee30bc1 ("KVM: x86: deliver KVM
IOAPIC scan request to target vCPUs")
Signed-off-by: Mao Wenan <maowenan@huawei.com>
Fixes: 7ee30bc1 ("KVM: x86: deliver KVM IOAPIC scan request to target vCPUs")
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>