powerpc/eeh: Fix build with STACKTRACE=n

The build breaks when STACKTRACE=n, eg. skiroot_defconfig:

  arch/powerpc/kernel/eeh_event.c:124:23: error: implicit declaration of function 'stack_trace_save'

Fix it with some ifdefs for now.

Fixes: 25baf3d8 ("powerpc/eeh: Defer printing stack trace")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>