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: 25baf3d816 ("powerpc/eeh: Defer printing stack trace")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/eeh_event.c

@@ -117,12 +117,14 @@ int __eeh_send_failure_event(struct eeh_pe *pe)
 	 * while the PE is sitting in the event queue.
 	 */
 	if (pe) {
+#ifdef CONFIG_STACKTRACE
 		/*
 		 * Save the current stack trace so we can dump it from the
 		 * event handler thread.
 		 */
 		pe->trace_entries = stack_trace_save(pe->stack_trace,
 					 ARRAY_SIZE(pe->stack_trace), 0);
+#endif /* CONFIG_STACKTRACE */
 
 		eeh_pe_state_mark(pe, EEH_PE_RECOVERING);
 	}