[POWERPC] Fix non-smp build

This fixes a compile error that only surfaces on CONFIG_SMP=n builds;
<asm/hvcall.h> seems to get pulled in through another header file for
SMP builds.  This problem was introduced by the hvcall stats patch.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>