PPC: fix sregs usage on booke

When compiling qemu with kvm support on BookE PPC machines, I get
the following error:

  cc1: warnings being treated as errors
  /tmp/qemu/target-ppc/kvm.c: In function 'kvm_arch_get_registers':
  /tmp/qemu/target-ppc/kvm.c:188: error: unused variable 'sregs'

This is due to overly ambitious #ifdef'ery introduced in 90dc88.
Fix it by keeping code that doesn't depend on new headers alive
for the compiler, but never executed due to failing capability
checks.

CC: Scott Wood <scottwood@freescale.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
