KVM: x86: Unexport x86_fpu_cache and make it static

Make x86_fpu_cache static now that FPU allocation and destruction is
handled entirely by common x86 code.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200608180218.20946-1-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>