powerpc: perf_event: Hide iseries_check_pending_irqs

If CONFIG_PPC_ISERIES isn't defined we end up with
iseries_check_pending_irqs and do_work at the same address.
perf ends up picking iseries_check_pending_irqs which creates
confusing backtraces.  Hide it.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>