KVM: SVM: Remove an unnecessary prototype declaration of sev_flush_asids()

Remove the forward declaration of sev_flush_asids(), which is only a few
lines above the function itself.

No functional change intended.

Reviewed by: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210422021125.3417167-15-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>