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/include/asm/eeh.h

@@ -89,6 +89,7 @@ struct eeh_pe {
 	struct list_head child;		/* Memb. child_list/eeh_phb_pe	*/
 	struct list_head edevs;		/* List of eeh_dev in this PE	*/
 
+#ifdef CONFIG_STACKTRACE
 	/*
 	 * Saved stack trace. When we find a PE freeze in eeh_dev_check_failure
 	 * the stack trace is saved here so we can print it in the recovery
@@ -99,6 +100,7 @@ struct eeh_pe {
 	 */
 	unsigned long stack_trace[64];
 	int trace_entries;
+#endif /* CONFIG_STACKTRACE */
 };
 
 #define eeh_pe_for_each_dev(pe, edev, tmp) \