KVM: powerpc: Fix BUILD_BUG_ON condition

The old BUILD_BUG_ON implementation didn't work with __builtin_constant_p().
Fixing that revealed this test had been inverted for a long time without
anybody noticing...

Signed-off-by: Hollis Blanchard <hollisb@us.ibm.com>
Signed-off-by: Avi Kivity <avi@redhat.com>