KVM: powerpc: Enable commented out BUILD_BUG_ON() assertion

KVM: powerpc: Enable commented out BUILD_BUG_ON() assertion

The BUILD_BUG_ON() assertion was commented out in commit 38634e676992
("powerpc/kvm: Remove problematic BUILD_BUG_ON statement") and fixed in
commit c0a187e12d48 ("KVM: powerpc: Fix BUILD_BUG_ON condition"), but
not enabled. Enable it now that this no longer breaks and remove the
comment.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250411084222.6916-1-thorsten.blum@linux.dev