KVM: arm64: Mark pkvm_pgtable_mm_ops static

It is not used outside of setup.c, mark it static.

Fixes:f320bc74 ("KVM: arm64: Prepare the creation of s1 mappings at EL2")

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Quentin Perret <qperret@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210514085640.3917886-2-qperret@google.com