kvm: move new trace event outside #ifdef CONFIG_KVM_ASYNC_PF

Fixes compilation with ppc64_defconfig.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>