KVM: arm64: Mark the host stage-2 memory pools static

The host stage-2 memory pools are not used outside of mem_protect.c,
mark them static.

Fixes: 1025c8c0 ("KVM: arm64: Wrap the host with a stage 2")
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/20210514085640.3917886-3-qperret@google.com