KVM: VMX: Fix header file dependency of asm/vmx.h

Include a definition of WARN_ON_ONCE() before using it.

Fixes: bb1fcc70

 ("KVM: nVMX: Allow L1 to use 5-level page walks for nested EPT")
Cc: Sean Christopherson <seanjc@google.com>
Signed-off-by: Jacob Xu <jacobhxu@google.com>
[reworded commit message; changed <asm/bug.h> to <linux/bug.h>]
Signed-off-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220225012959.1554168-1-jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>