powerpc: Remove export of save_stack_trace_tsk_reliable()

As tglx points out, there are no in-tree module users of
save_stack_trace_tsk_reliable() and its x86 counterpart is not
exported, so remove the powerpc symbol export.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>