KVM: x86/mmu: Make tdp_mmu_allowed static

Make tdp_mmu_allowed static since it is only ever used within
arch/x86/kvm/mmu/mmu.c.

Link: https://lore.kernel.org/kvm/202302072055.odjDVd5V-lkp@intel.com/


Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20230213212844.3062733-1-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>