KVM: arm64: Add break to default case in kvm_pgtable_stage2_pte_prot()

KVM: arm64: Add break to default case in kvm_pgtable_stage2_pte_prot()

Clang warns (or errors with CONFIG_WERROR=y / W=e):

  arch/arm64/kvm/hyp/pgtable.c:757:2: error: label at end of compound statement is a C23 extension [-Werror,-Wc23-extensions]
    757 |         }
        |         ^

With older versions of clang (15 and older) and GCC (at least the minimum
supported, 8.1), this is an unconditional hard error:

  arch/arm64/kvm/hyp/pgtable.c: In function 'kvm_pgtable_stage2_pte_prot':
  arch/arm64/kvm/hyp/pgtable.c:756:2: error: label at end of compound statement
    default:
    ^~~~~~~

  arch/arm64/kvm/hyp/pgtable.c:756:10: error: label at end of compound statement: expected statement
          default:
                  ^
                   ;

Add a break statement to this default case to clear up the error/warning.

Fixes: 2608563b466b ("KVM: arm64: Add support for FEAT_XNX stage-2 permissions")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://msgid.link/20251125-arm64-kvm-hyp-pgtable-fix-c23-ext-warn-v1-1-98b506ddefbf@kernel.org
Signed-off-by: Oliver Upton <oupton@kernel.org>